[MAYA] On January 21st, 1925, Dr.

Curtis  Welch entered his clinic in Nome, Alaska, sat down, and put his head in his hands. Two children were dead, and more would follow.

Earlier that month, he had admitted a 3-year-old  boy named Billy Barnett with a sore throat, fever, and fatigue. Welch hoped it was tonsillitis. But Billy soon developed thick,  gray lesions in his throat that were the hallmarks of another disease.

And while initial treatments had seemed to work, by 4 PM on January 20th, Billy  started coughing up blood. By 6 PM, his airway began to close up. Late that evening, Welch could do nothing  but watch the boy die, struggling for air.

And on the morning of the 21st, Welch  woke up to even worse news: another child, a 7-year-old Inuit girl named Bessie  Stanley had identical symptoms. Once again, Welch could do nothing to save her. Both children’s symptoms matched a particular  disease: the “Strangling Angel”.

Diphtheria. And with two separate cases  back-to-back, this was no coincidence. This was the start of an outbreak  in the tiny town of Nome.

Isolated from the rest of the world, with  the next closest doctor 10 days away, every child in the town and  surrounding villages was in danger. But this story has a happy  ending, relatively speaking, thanks to an incredible, death-defying journey by a relay race of sled dogs  and the people driving them. It was so incredible, it’s been  immortalized in several forms.

One of the dogs got taxidermied  and put in a Cleveland museum, and Steven Spielberg’s production company made  an animated kids movie about it back in 1995. Yes, I am talking about Balto,  both the taxidermy and the movie. If you’re of a certain age, there’s a good  chance you’ve seen that film at least once.

Your parents might even still  have the VHS in their basement. But SciShow’s editorial team re-watched  it while writing this episode, and I hate to break it to you,  but it’s almost entirely made-up. There were, for example, no talking polar bears.

So what really happened? How did people get life-saving medication to the  children of Nome in the dead of winter, 1925? I’m Maya Higa, and I share stories from  my animal sanctuary every week on YouTube.

A lot of amazing things  happen at Alveus Sanctuary, but thankfully, nothing quite as  dramatic as the tale I’m about to tell. I’m going to introduce you to the real Balto. And along the way, we’re  going to meet the sled dogs that have been a critical part of human  life in the Arctic for thousands of years. [♪ INTRO] [♪ MUSIC] Located roughly 230 kilometers from the  Arctic circle, on Alaska’s Seward Peninsula, Nome is one of the most isolated  cities in North America.

Well, “city” is probably the wrong word. Originally a settlement by the local indigenous  people, known as Inupiat or Alaskan Inuit, Nome’s population surged to a whopping  20,000 people in the summer of 1901, a few years after gold was  discovered in a nearby creek. But by 1925, when Dr.

Welch’s story  started, Nome had already become a relic. A sleepy settlement of about 1,400. The wooden houses, churches, and  offices strung up with telephone   wires were set against the treeless Arctic tundra.

And, of course, there were a bunch of dogs. [BRAVERMAN] The thing to  understand about Nome at that time, and really a lot of northern places at that time, is that sled dogs, in the same way that  our society now is built around cars, Nome at the time was built around sled dogs. [MAYA] That’s Blair Braverman, a  long-distance dog sledder and writer. [BRAVERMAN] I got into dog  sledding when I was a teenager. I graduated high school in California and I   immediately moved to the Norwegian  Arctic to become a dog sledder. And that was 19 years ago.  I've been doing it ever since.

I've done some of the biggest dog sled races in  the world. 200 miles, 400 miles, a thousand miles. I've done the Iditarod. I've done the Kobuk  440, the Canadian Challenge, the Bear Grease.

Just some really big dog sled races. The first time I heard about the Serum Run, I was a kid in California and I had a little  book called Balto, the Bravest Dog Ever. And I loved it.

Like I read it to shreds I sort of credit that book for why I  got into mushing because as I got older,   I started reading other dog sledding books and I realized it wasn't just  a thing that belonged to the   past. It was a thing that people were doing now. And it involved dogs and it  involved wilderness and it involved teamwork and self-reliance and   building this incredible trust and  communication with your team of dogs.

And I was like, that is too good to be  true, like can I really pursue that? [MAYA] And like the stories suggested, Nome  really was kind of a dog-centric place. [BRAVERMAN] Sled dogs were… they  would go into bars with people. That's how you would get from roadhouse  to roadhouse or go to your friend's house. Everyone had sled dogs in their yard They all had their own teams.

And  these weren't, beasts of burden. They were just incredibly beloved  key members of the community. [MAYA] And with the dogs came mushers:  the people who drive the dog sleds. Among them was Leonhard Seppala, who lived in  town with his wife and 8-year-old daughter. [BRAVERMAN] Seppala was a  legendary musher at the time.

I know he and his lead dog Togo had mushed like   55,000 miles together. Like they  had just dealt with everything. He knew the trails.

He knew how to cross sea ice. He knew how to deal with wildlife, the depth  of understanding that he had with his dogs, the communication he was able to have with them, the way he was able to help  his dogs have what they needed to do the most extraordinary things with  him was really unparalleled at the time. [MAYA] In 1925, Seppala was already  famous for winning many sled dog races. But his official job was working for a local  gold mine company as their main sled dog musher, helping ferry people and  supplies between dig sites.

Among the mine’s kennels were two dogs who  would each get movies named after them: There was Togo, who was played  by one of his descendents acting alongside Willem Dafoe in a  2019 live action movie for Disney+. There was also Balto, because  unlike the animated movie, Balto was not a wild wolfdog living  in a shipwreck with a Russian goose. And who continued living in that  shipwreck for two direct-to-video sequels.

Now, it’s important to note that, up in Nome, dogsleds weren’t just a way for folks  to get around town and go to work. They were also one of the town’s few  connections to the outside world. There were no roads or railways out of  town.

Instead, people relied on two options. One was a system of dog sled trails that stretched   over 1,000 kilometers east to  the railroad town of Nenana. This route was studded with other small  towns and the occasional rest house,   and took about 25 days to cross.

In the winter, it was the only way  to transport mail and supplies. But dog sleds can only carry so much weight. So during the warmer months of the year, much of the traffic to Nome  came the second way: steamships.

As long as Nome’s port was ice-free, these  could chug up from ports like Anchorage to dock at the harbor and offload  supplies or the occasional visitor. And while we can’t say for certain, it’s on  such a ship that diphtheria may have travelled. Diseases weren’t uncommon out on the frontier,  but diphtheria was particularly deadly. [TRUELOVE] Back in the early 1900s,   diphtheria was I think around the fourth or  sixth highest cause of death in the U.

S., with most of those deaths occurring  in children under five years old. [MAYA] That’s Shaun Truelove,   an epidemiologist and infectious disease  researcher at Johns Hopkins University. He studies disease outbreaks and how we might  find new ways to prevent them in the future. [TRUELOVE] I first learned about  the serum run… maybe it was about   that - thinking about the movie Balto, right? But really, you know, the first time I  really understood the details of it was   during this re-review and analysis of diphtheria. [MAYA] In 2020, Truelove and some colleagues  wrote a major paper that rounded-up what we knew about how diphtheria  attacks the body and spreads. [TRUELOVE] You look at some of the  papers and some of the common knowledge that we have really kind of relied  on for response to these outbreaks, I, you know, we realized that  it was quite out of date.

Because we really hadn't thought  about this disease for a long time. [MAYA] Though diphtheria is rare today, back  in Dr. Welch’s time, it was a major killer. During an epidemic in 1921, the United States  had more than 200,000 cases, and 15,000 deaths.

It was especially deadly for little kids, with a  20% mortality rate for anyone under 5 years old. For it to show up a few years later in  the tiny town of Nome was terrifying. As for the actual culprit behind the disease, we’re dealing with a bacteria  called Corynebacterium diphtheriae. [TRUELOVE] When someone gets diphtheria, the disease progression looks a lot like other  bacterial respiratory tract infections for a time.

There’ll be kind of the typical runny nose,  sore throat, cough, that kind of thing. But as the disease progresses in a lot of cases, patients or cases will develop  what's called membranous diphtheria, where they'll develop a membrane in their throat  that actually ends up obstructing their breathing. And that's one of the leading causes of death. [MAYA] This gray, leathery, foul-smelling  membrane is the terrible hallmark that Welch had seen in the throats of the  first Nome children who had gotten sick.

It’s made of dead throat tissue cells,  killed by a toxin the bacteria secrete. Although oddly, the bacteria only secrete  the stuff if they are also infected this time, with a bacteria-targeting virus. That’s right, diphtheria is so bad, even  the bacteria that cause diphtheria are sick!

The technical term for one of  these viruses is a bacteriophage. And one of the things bacteriophages do is  they insert bits of their own genetic code into the DNA of the bacterial cell they infect. They basically hijack the cell  and force it to start making a   bunch of proteins so the virus can replicate.

In C. diphtheriae’s case, one of those  proteins turns out to be toxic for humans,   chickens, guinea pigs, and more. Scientists just call it diphtheria toxin, or DT. Today, there are three potential  tools for dealing with diphtheria.

The first is a vaccine. It was first rolled out   across the U. S. a few years after  that 1921 epidemic I mentioned.

But it’s not clear how much, if any, vaccine got  all the way to Nome. The second is antibiotics. [TRUELOVE] We have three that work against  it, penicillin, erythromycin, and clindamycin. But those weren't developed.

Penicillin wasn't   discovered until 1928. So this  is well after the Nome incident. [MAYA] So at the time of the Nome outbreak, they  really only had the third option: antitoxin. An antitoxin is kind of like how  we have antivenom for snakebites, just instead of snake venom,  it’s that bacterial toxin. [TRUELOVE] So it neutralizes  the toxin.

And with diphtheria,   all of the disease - well, I can’t say all of it. Most of the disease, especially the severe  disease, that’s caused by diphtheria,   is caused by the toxin that it secretes. So that was developed starting in  the eighteen nineties.

And what's   actually super interesting and funny about this, the whole process to develop  antitoxin for diphtheria   relies on us to inject horses with the toxin and then extract this molecule that the  horse developed to neutralize the toxin. We still use that same process today. [MAYA] The molecule that Truelove is talking about  is an antibody made by the horse’s immune system, in response to getting poisoned. Scientists simply had to collect blood from the  horse, and separate out the antibody-rich serum.

Now, this antitoxin isn’t a silver bullet. It doesn’t kill the bacteria,  doesn’t stop the spread,   and doesn’t heal the damage  the toxin’s already done. But if administered early enough, it can help  prevent further damage and give the patient time for their own immune system to do its thing.

By 1925, it was common for doctors,   even doctors in very remote areas,  to have some antitoxin on hand. But up in Nome, there was a problem. The year before, in 1924, Dr.

Welch  had noticed his supply had expired. And while he had ordered a replacement  batch, the package never came. After he realized he was dealing with diphtheria,   Welch tried giving some of the  expired antitoxin to Bessie Stanley.

But it didn’t work. Maybe she was too far gone  by the time he got there. Maybe, more than a year past its  expiration date, it had lost its potency.

But even if the antitoxin had worked, he  only had enough for maybe six people at most. And while 1,400 is not a large  number as far as town populations go, it’s definitely more than six. So as soon as Welch got back from seeing Bessie,  he picked up the phone and called Nome’s mayor.

Together, they gathered the town  council and came up with a plan. The first step was quarantine. Immediately.

They even sent someone to go shut down a  card game party that was going on that night. That would hopefully stop  the spread of the disease, but it wouldn’t save the who-knows-how-many  people who were already infected. Then, Welch and the mayor sent a desperate call   for more antitoxin down the telegraph  lines.

The US government responded: if they pooled together everything  they could possibly spare, they could send about 1.1  million units of the antitoxin. That sounds like a lot. But  1.1 million units was only   enough to treat maybe 50 cases of diphtheria, max.

Units are a measurement of the antitoxin’s  potency, not a number of doses. Hopefully that much would be enough  to cover all the severe infections. But it would take time to round up  the medicine and send it northwards.

There was a bit of good news, though: 300,000 additional units were found  forgotten and stored in Anchorage. Which, while over 800 kilometers away as  the crow flies, was at least in Alaska. While that wasn’t nearly as much as Welch wanted, it would save those in mortal peril and buy  time for the larger stockpile to arrive.

But this small supply still had to get to Nome. The question was, how? It was January; the harbor was frozen  over.

So a ship was out of the question. The mayor wanted to take advantage of an emerging  technology and send the shipment by airplane. But a variety of problems, like the fact  that antifreeze hadn’t been invented yet, meant it was way too dangerous.

So there was only one answer to that question. And if you’ve picked up on the incredibly subtle  foreshadowing we’ve been laying down so far, you already know what that answer is. [♪ MUSIC] [MAYA] In contrast to the airplane, dog sledding may actually be one  of humanity's oldest technologies. [BRAVERMAN] The history of sled dogs in the Arctic   goes back almost as far as the  history of people in the Arctic. [MAYA] In 1989 and 1990, on  Zhokhov Island in Siberia, archaeologists uncovered the remains  of what seem to be domesticated dogs. They’re roughly 9,500 years old. [HUSON] Not only were there dog remains there,   but there was also evidence of sleds  and harnesses that were still there.

So they knew to a pretty good extent that these  must have been used for sledding purposes. [MAYA] That’s Heather Huson,   an associate professor at Cornell  University who studies animal genetics. Some of that research includes  looking at working dog breeds, and investigating how they’ve  evolved over centuries,   if not millennia, of artificial selection. And like Blair Braverman, Huson also has  first-hand experience with racing sled dogs. [HUSON] Both of my parents raced sled dogs.

I started racing when I was about eight years old. We would train together and we would travel every   weekend in the winter to different  races around the US and Canada. I went to Alaska when I was first a  teenager and I raced up there in Fairbanks.

And I lived up there for six years,  raced dogs, worked, did research. And I had every intention of staying there except  for work brought me back to the East coast. [MAYA] As part of that work, Huson learned one  particularly cool thing about sled dog genetics. There are official breeds of dogs, like  Siberian Huskies or Alaskan Malamutes, that have been selectively bred for a  specific look and can be used as sled dogs.

But when you look at the Alaskan sled dogs  that actually pull many of these teams, they’re not quite as copy-paste. They’re a motley crew of different colors,  coat lengths, ear shape. Just a rag-tag team. [HUSON] When I started my PhD, I  was doing the genetics of sled dogs.

And one of my questions  was, what is their ancestry? You know, how much is a Siberian Husky  versus an Alaskan Malamute or a Greyhound? And the big thing that we found  was sled dogs are their own breed. [MAYA] So even though they aren't  a recognized registered breed, they've been selected for for the last  hundred years just like every other breed.

But they're selected for their performance. These hard working good boys and girls have  been bred over generations not for looks, but for their sled-hauling,  cold-enduring abilities. And this selection has modified  their DNA in detectable ways. [HUSON] One of the really cool  mutations that has been explored in dogs through domestication and across  species is the amylase gene.

So amylase produces basically a protein  or an enzyme that digests starch. So starch is a big part of our diet nowadays. But if you think historically, you  know, if we think about a wolf, you know, the primary diet of a wolf  is going to be meat and meat proteins.

For a wolf, they typically will have  about two copies of this amylase gene. And then what has been found is Arctic breed   dogs have about seven to 10  copies of this amylase gene, where basically every other dog breed has  up to 20 copies or so of this amylase gene. So there is a huge difference between  how many copies of the amylase gene from wolves to Arctic dogs  to basically every other dog.

Arctic peoples eat a very high meat protein diet  and their dogs are eating that same type of diet. Whereas all of the other dogs they grew up as   people you know evolved and had  more farming and agriculture, those dogs were exposed to more  of those types of foods as well. So their diets had more starch in them. [MAYA] Another cool genetics revelation is  that scientists have tied modern sled dogs all the way back to those Zhokov Island animals,   meaning those ancient dogs  may be their direct ancestors.

As for when sled dogs got to Alaska, it seems like they’ve been running around  North America for at least 1,000 years. While there were some dogs in the  North American Arctic before this, archaeological evidence suggests they were  relatively rare and not used for sledding. Not until the ancestors of today’s  Inuit emerged in Western Alaska.

This group, called the Thule  culture, began to spread out and eventually displace the people known  as the Dorset about 1,000 years ago, probably assisted by their invention of  the kayak and use of the dog sled on land. Hundreds of years later, when Alaska’s  gold rush hit, sled dogs proved essential. [BRAVERMAN] During the gold rush, people  brought every animal they could think of   that might be able to transport things. But sled dogs are what worked. [MAYA] In fact, it was said the  demand for sled dogs was so high, people would kidnap sturdy-looking dogs off the   streets of any West Coast port  town and send them to Alaska.

Then that diverse group of dogs  would breed the next generation. Huson has seen evidence of this mixing. One notable piece of evidence: Balto himself!

Because while Balto died back in 1933,  he’s still with us…in taxidermied form. [HUSON] I was talking to a veterinarian,   Jerry Van Eck, who is a sled dog  veterinarian and a history buff. And he put out the opportunity, would I  be interested in sequencing Balto's DNA? And of course I was like,  why not?

This sounds cool. Jerry knew some of the curators at the  Cleveland Museum of Natural History. And the Cleveland Museum is where Balto's  hide has been preserved for years.

They were willing to provide a  small amount of skin and hair   sample from Balto's belly on his preserved hide and that’s how we ended up sequencing Balto’s DNA. One of the highlights in sequencing Balto's  genome was understanding that, you know, he had less deleterious or negative mutations  than a lot of our modern breed dogs do now. And that was probably because the dogs then, they were a much more diverse population of dogs. [MAYA] But you know what Huson  and her team didn’t find? [HUSON] One of the things that we could  very clearly identify using Balto's genome, Balto was not a wolf hybrid.  So there was no wolf in Balto.

Balto was definitely a domesticated dog. [MAYA] That’s right; the entire reason why  movie-Balto was such an outcast from society, forced to sleep on a shipwreck  with a Russian Goose, wasn’t true. Okay, to be fair, the paper announcing this  genetic discovery was published in 2023, which is nine years after the  last Balto movie came out. And to be clear, they didn’t pull  this falsehood out of nowhere.

Some historical accounts also  made the half-wolf claim, and there are accounts that Inuit or other people   in Alaska may have encouraged  hybridization with wolves. But as Huson told us, “Balto was  definitely a domesticated dog.” In fact, she thinks hybridization  probably wasn’t super common. While wolves are tough, social animals,   they’re not as good as sled  dogs at running long distances.

And even if they are more social,  they can also be more conflict-prone. [HUSON] Could it have happened maybe  intentionally or unintentionally? Maybe occasionally, who knows? But is it really common and  is it very intentional?

No. [MAYA] Now, thanks to this long history  of living and working in the Arctic, sled dogs have a number of physical  adaptations that make them champions   at surviving through cold conditions. [BRAVERMAN] Sled dogs have double coats. So they have long guard hairs that function  almost as like a windbreaker or a raincoat. They keep moisture out.

And then they have  really fluffy downy fur underneath that. So it's a super insulating dual  layer fur that is able to keep   them completely warm in really cold temperatures. Sometimes it'll be like 10 below and I'll go  outside, and none of my dogs are in their houses.

Like every single one of them  is just lounging in the snow and there'll be snow falling on them  and they're just rolling over and like, putting their belly up and just think it's  the most comfortable thing in the world. [MAYA] Scientists have also found changes in genes   related to their ability to  sense temperature and pain, as well as how much oxygen their muscles need. And, in 2005, scientists discovered that some  sled dogs have another remarkable adaptation, which helps them run for hours  without ever getting tired. [BRAVERMAN] After a couple hundred miles of  dog sledding, you can feel the dogs change. They go into this mode where they're  sort of just able to go forever.

They're not getting tired.  They're not slowing down. They're just like, it's almost  machine-like, and it's very uncanny to see. Scientists call that flipping a metabolic  switch, and mushers call it going trail hard. [MAYA] The researchers who uncovered this switch  ran a series of 160 kilometer-long mock sled races and took tiny samples of the  dogs’ leg muscles after each one.

For the first few days, it  was clear that the muscles   were accessing the energy stores inside of them. Specifically, sugar  macromolecules called glycogen. That’s basically the same  thing that our own muscles do.

But rather than bleeding those glycogen stores  dry, and leaving the dogs utterly exhausted, the muscles eventually switched to  using something outside of them. The researchers behind that 2005  discovery proposed the muscles could be pulling fat molecules out of the  blood and burning them, instead. However, more recent research argues that while   fat does play an important  role in preventing fatigue, the dogs aren’t burning that fat directly.

For example, a paper from 2021 hypothesizes it’s  a complicated process that starts in the liver, featuring a bunch of different  molecules including fats and sugars. Which we don’t have time to get into right  now, because we have to get back to Nome. To make the long journey through the Alaskan  wilderness and deliver the antitoxin, Nome’s sled dogs were going to  need all of these adaptations.

Because they were about to attempt  something few animals could survive. [♪ MUSIC] Fans of the movie Balto may recall that  the dog Balto wasn’t actually on the team that set out from Nome to pick up  the serum in the town of Nenana. And that much is true. In reality, the  serum run was a thousand kilometer relay.

As in, multiple mushers and  multiple teams of sled dogs. The Governor of Alaska sent messages  to Nome, the post office in Nenana, and a trading company that also did mail delivery   to find the best mushers they could  along the route, and set-up a relay. Only then would he send the  serum northwards by train.

Up and down the trail, mushers, some of whom had just finished runs of their  own, or who were struggling with poverty, or were the renowned racer Leonhard Seppala, stepped up for this volunteer mission. And when that train got to Nenana, on the evening of January 27th, the first musher, a mail-runner, miner, and  trapper named Wild Bill Shannon, was already there ready to meet it. A more cautious man would have waited  for morning to depart with the antitoxin.

But Wild Bill, true to his  name, set off that same night, reportedly saying: “Hell… if people  are dying… let’s get started.” He might have regretted that decision. As a general rule at the time,  sledders wouldn’t go out if the   temperature was below -40 degrees Celsius. But for this time of year and place,  temperatures on the ground could be -50 Celsius.

And if you’ve never been in this kind  of weather, it’s incomprehensibly cold. Just a few years earlier, in similar conditions, a trapper lost both of his feet to frostbite  after accidentally getting his boots wet. By the time he was able to  run, then walk, then hobble two-and-a-half kilometers to the nearest  telegraph station, his feet were beyond saving. [BRAVERMAN] The amount of clothing  you need to be mushing in 20, 30,   50 below is so categorically different from anything you'd wear when you're in a city   and you're going ultimately  from one building to another.

It's like you're carrying a  building with you around your body, So I want to show you um my mushing  mittens, and these are very typical. Every musher has mittens like this. Some of them are made out of different fur.

I made these out of beavers that my dogs ate. This would be sort of like standard mushing wear. These have nipples. [MAYA] But the biting cold wasn’t  the only foe these teams had to face.

There were all kinds of hazards along the  trail, like thin ice over frozen rivers. And between the dark and the falling snow,  there was often almost zero visibility. But ultimately, the biggest threat  during the Nome Serum Run was time.

Normally, the sled dog route between Nenana  and Nome would take something like 25 days. And if the weather turned really bad, well, the mushers could just hunker down  where they were and wait it out,   even if they were out on the trail. [Braverman] You can basically just hole up in a  snow cave at any time and wait out the weather. So what made the serum run different is that  they were not able to do that because they knew that the lives of children depended on their  ability to push as hard as they possibly could. [MAYA] Even if the children back  in Nome could wait that delay out,   their life-saving medicine couldn’t.

The serum was kept in glass vials. So  it couldn’t be allowed to freeze solid or it’d expand and break the  glass, ruining the batch. The solution that the mushers implemented  was relatively straightforward: bundle the package in heavy insulation,  and just book it to the next roadhouse, which were kind of like small log-cabin  inns or rest stops with a stove inside, where they could re-warm the  vials in front of the fire. [BRAVERMAN] They were pushing through  conditions that went against their instincts to mush in because they had no choice. they were trying to save  the lives of these children. [MAYA] While Shannon could brush off  the cold at the start of his journey, eventually his limbs and face became  numb, and his movements grew sluggish.

At one point, he even jumped off the sled and  jogged alongside the dogs in an effort to warm up. But it wasn’t enough. Shannon  was becoming hypothermic.

Hypothermia is a dangerous  drop in body temperature. To conserve heat around the  important internal organs…the brain,   the heart… you know, the classics… the body will shunt blood  away from the extremities. Early symptoms can include shivering and increased  heart-rate as the body tries to warm itself, but soon muscles become hard to control.

Balance suffers. Memory and  judgment become impaired as   the brain starts to cool and blood flow slows. In severe hypothermia, people have  even been known to act irrationally, walking in circles or even  taking off their clothes.

Eventually they go into a kind of stupor,  curl up, fall unconscious, and die. When Shannon finally stumbled into the  roadhouse in the town of Minto, it was 3 AM. He’d gone 48 kilometers.

It was negative 52 degrees Celsius. Parts  of his face had turned black from frostbite. According to some reports it was weeks before  he could touch his face without feeling pain.

Even some of his dogs, for all their super  adaptations to the cold, also had frostbite. This night had pushed them to their limits. Some also had blood in their mouths, from  what mushers back then called “lung scorch.” This is likely what today’s doctors  would call pulmonary hemorrhage.

When it’s colder than -50 degrees Celsius, even breathing in air can be dangerous. It’s so cold, it irritates and  damages the inside of the airways. And yes, it can be fatal.

After resting a few hours and allowing  the serum to re-warm by the fire, Bill Shannon, his face still black with frostbite, went back out into the cold to  complete his leg of the journey. He left three of his dogs behind,  all of whom died from their injuries. Shannon would mush another 35 kilometers and pass off the serum to Edgar  Kallands in the town of Tolovana around 11 AM on January 28th.

In total, Wild Bill Shannon had gone  roughly 83 kilometers in just over 12 hours. The first leg of the race was over. But before we get to the longest  and arguably most dangerous leg, let’s check back in with Nome.

The tiny Alaskan town had entered  quarantine, as Dr. Welch requested. [TRUELOVE] Diphtheria spreads through  respiratory transmission, generally speaking. And that's really what we usually worry about.

It really depends on the setting and the  situation, just how transmissible it is. But we do think it has a reproductive  number of somewhere in the twos to fours. [MAYA] This means that each sick person might   typically go on to infect  two to four other people. But in Nome, it may have been even worse. [TRUELOVE] You're talking also about  a situation where people are indoors.

I imagine, you know, I wasn't around then, but they're huddled in space  where they're trying to keep warm. And so you have a lot of  potential for transmission. [MAYA] Welch couldn’t know how  many people were already sick. He was especially concerned  about the villages around Nome,   particularly ones where the Inuit people lived. [TRUELOVE] You know, as we've  seen throughout the history   of the colonization of the Americas, right?

The European settlers and populations often had   immunity to a lot of the pathogens  that we are, you know, used to. Unfortunately, for the indigenous  populations around Nome, most likely they had never been exposed, right? So it wouldn't just have been the  children at high risk for diphtheria, potentially, it could have also been, you know,  the full population, the adults, the elderly. [MAYA] More people were starting to show symptoms, including Billy Barnett’s five-year-old sister, and several members of Bessie Stanley’s family.   On top of that, two other  adult men had sore throats.

But were these an early sign of  diphtheria, or something less dangerous? Welch prescribed thousands of units from  his limited supply of expired antitoxin, hoping it had any power left. Some of his  patients were improving, some weren’t.

They needed the fresh antitoxin. [♪ MUSIC] The morning Bill Shannon passed the proverbial  baton, it was time for Leonhard Seppala to set out for his scheduled rendezvous point  in the town of Nulato. A 500 kilometer journey. At the time, the plan was to pick up the serum and  turn around and do the entire 500 kilometers back.

In other words, Seppala was supposed  to run roughly half of the Serum Run all by himself…well, him and his dogs. Before he left Nome, Seppala left explicit  instructions to his co-worker, Gunnar Kaasen. [BRAVERMAN] It's super, super common for mushers  to have handlers or sort of younger mushers who are running their dogs, who  are working at their kennel. So there really is like an  apprenticeship system in the sport.

So Gunnar, you know, was a musher, but  he was mushing with Seppala's dogs. [MAYA] Seppala’s order was  that, if Kaasen needed the dogs,   he could take them, but to  not use Balto as the leader. Another dog, Fox, was Seppala’s choice. We don’t know Seppala’s exact reasoning, but we know that Balto was  inexperienced and relatively slow,   not used for fast relays but for hauling freight.

Seppala likely just thought  Fox was a better lead dog. [BRAVERMAN] So the lead dog runs  at the very front of the team, and they have a couple responsibilities   that are a lot bigger than the  responsibilities of the other dogs. It's their job to find the trail Wherever the lead dog goes, the  rest of the team will follow. So if a musher has a command, if they're going to say, turn right, turn left, don't chase that reindeer, whatever  they're trying to tell their dogs, the lead dog is the one they're talking to.

The other thing is that  sled dogs are pack animals. So they like following, they like  chasing the dog in front of them. And so it's psychologically a lot harder  for a sled dog to be in front of the team where they have an open trail  in front of them and they're   not chasing another dog's butt, so to speak.

It just is more psychologically  challenging. It takes more confidence. It takes more independence.

And so being a lead dog isn't even  something we can teach dogs, really. It's something they either are or aren't. And a lot of great sled dogs never,  ever, ever run in lead because they just psychologically aren't  a great fit for that role. [MAYA] Now, as I mentioned, The Powers That Be originally wanted Seppala  to do more than half of the relay by himself.

But after he left on January 28th,  the outbreak picked up speed in Nome. More kids were getting sick. And even a champion racer would need to pace  himself in order to cover 1,000 kilometers.

So the governor ordered more  mushers to be brought on, hoping more shorter legs and more  fresh runners would speed up the pace. With Seppala already gone,  they had no way to contact him. They just had to hope one of  the other mushers would stop   him along the trail and tell him the new plan.

Seppala would no longer go all the way to Nulato, nor would he be the one to  bring the serum to Nome. Nevertheless, Seppala was still assigned the  longest and most dangerous stretch of the run. Over the following three days,  as Seppala raced eastward, 16 mushers brought the life-saving package  another 730 kilometers or so closer to Nome.

Henry Ivanoff was the one to catch  Seppala outside the town of Shaktoolik, tell him “Change of plans!”,  and hand-off the medicine. And now, Seppala had to make a choice. In order to get to Shaktoolik in good time, he’d taken a shortcut across  the frozen-over Norton Sound.

In other words, he wasn’t mushing  on solid ground, but across sea ice. To meet up with the next musher in the relay, Seppala could take the safer but much longer  route around the sound, hugging the shore. Or, he could go back across the way he came  and cross roughly 35 kilometers of sea ice.

While crossing the ice had been  relatively easy, it was now getting dark, and the winds were getting stronger. Exposed to the elements, the wind  chill could reach -65 degrees Celsius. And just because he and his dogs  made it safely across the ice once, didn’t mean they’d do so going back.

Seppala knew only too well what  could happen crossing sea ice. On a previous, less important run, he’d been a few kilometers offshore when  the ice beneath him suddenly fractured stranding him, his dogs, and  his whole sled on a little ice   island with a three-meter-gap  between him and the ice sheet. It looked like he’d be stuck on the ice flow until either the winds changed or the ice  broke apart and he drowned in the Arctic Ocean.

But Seppala had a plan. His idea  was to tie a rope to his lead dog, Togo, throw him across the gap,  then order the dog to run forward. Hopefully, Togo would be strong enough  to pull the floating ice back together.

This was a huge task, but  Seppala had faith in Togo. And as lead dog, he was the  best chance Seppala had. So Seppala threw Togo, and the rope came undone.

But it turns out, Togo was  the goodest of good boys. He dove into the icy water,  found the end of the rope, and pulled the ice back together  with the power of his teeth… plus whatever friction his paws  had against the ice below them. [BRAVerMAN] The story about Togo saving the  team from an ice flow is 100% plausible to me after two decades in mushing.  I have had a couple dogs on my   team who have saved my life a handful of times. I wasn't floating away on an ice flow, but  there were some pretty dire situations where individual dogs sort of came up with a solution  and then led the entire team through it. [MAYA] There’s no one way for  an animal to be intelligent.

Dogs, in general, seem to be  very good problem-solvers, although it depends on  exactly what that problem is. For example, they do really well with  inferring information based on social cues, especially if those cues come from a human. So if a human points at an object, the dog  can understand that the human wants it.

They’re even better than chimpanzees at this. On the other hand, dogs aren’t great at  means-end experiments in laboratory settings, where they’re tested on the connection between  things like “pull rope” and “get treat.” But lucky for Seppala, he was not a treat. He was a person with whom Togo  had a very close relationship,   and the social cues likely came into play.

So they both lived to tell the  tale, and on January 31st 1925,   Seppala found himself in a decision-making pickle: With serum in hand, he had to decide whether the  risk of an ice break happening again was worth it. And in the end, he chose the riskier path. He and his dogs, Togo included,  would cross the frozen Sound.

But given the hour and the miserable  weather, he couldn’t see where he was going. Seppala had to trust Togo was smart  enough to find the way home on his own, and once again problem-solve  their way out of any danger. Seppala arrived at the roadhouse in  Isaac’s Point several hours later.

There was still another  stretch of sea ice to cross which would keep him and his dogs closer to shore, and then a whole mountain to traverse  before reaching the next musher. But the most dangerous section was behind them. Seppala eventually handed the  serum off to a musher named Olson, who then, 40 kilometers  later at the town of Bluff, handed it off to Seppala’s  coworker, Gunnar Kaasen.

Kaasen wasn’t originally going  to take part in the relay at all. But when the governor put the call out for  more mushers, they tapped him to take part. This meant Kaasen had to take the gold mine’s  dogs out, which is to say, Seppala’s dogs.

But it wasn’t Fox that Kaasen  had chosen for his lead dog,   per Seppala’s instructions. It was Balto. [♪ MUSIC] We don’t know why Kaasen went  against Seppala’s instructions. Disobeying another musher’s orders on how to  take care of their dogs was a major faux pas, but perhaps he saw something in the  dog he thought Seppala had missed.

It’s a good thing Seppala was wrong about Balto, because by this point in our story,  the weather was just terrible. Wind gusts had reached 112 kilometers an hour, just under the speed you need for the  National Weather Service to upgrade a   cyclone to a hurricane. It was a proper blizzard.

The weather was so bad, in fact,  that sometime after Kaasen left Nome, Dr. Welch actually tried to call off the  relay. If any of the mushers got lost like what happened in the movie, the serum would freeze and be ruined, like what should have happened in the movie.

Nome’s mayor called ahead to  the next roadhouse in Solomon, and told them to stop Kaasen when they saw him. Someone also called Ed Rohn,  the final musher in the relay,   and told him the race was  paused because of the storm. But Kaasen never got the message.

Based on his later actions,   it seems like he would have ignored  that order even if he had received it. With visibility so low, Kaasen  and his dogs had to depend on   Balto’s nose and keen senses to stay on the trail, detecting the faint scent of  past dogs that had run the route. As Huson and her colleagues showed  the world in their 2023 research, Balto didn’t need to be part wolf to do this.

Domesticated dogs have a very powerful  sense of smell all on their own, more than a thousand times better than a human’s. But finding the trail was only  one part of the challenge. The team still had to brave the elements.

Along the way, they evaded  hazards like overflowing   rivers that would have frozen the dog’s feet, and crawled up a mountain  cliffside trail on their bellies. But coming down that mountain, disaster struck. Kaasen and the dogs passed through a  sort of naturally-occuring wind tunnel.

Basically, a valley that could channel  the wind down a narrow passageway,   and create some really intense gusts. So intense, in fact, that they lifted the sled   off its runners and sent it and the  dogs careening into the snow drifts. And during the last gust he encountered, Kaasen realized the serum  package had gone missing.

He dropped to his hands and knees,  frantic, grabbing bare-handed at the snow, only to thankfully find it a moment or so later. He lashed it to the sled,  and got back on the trail. Then, after finally hitting a good stretch with  the wind at their backs to help push them along, Kaasen made his next controversial move.

When he arrived at the roadhouse where the  supposed-to-be-final musher was waiting, the lights were out. Kaasen  assumed the musher was sleeping. And instead of waking him and  handing off the antitoxin, Kaasen pushed on toward Nome  to finish the job himself.

It was a dangerous move given how  many kilometers he had already logged. But Kaasen, Balto, and the  rest of the dogs arrived in   Nome shortly before 6 AM on February 2nd, just five and a half days after the first team  set out on a journey that usually takes 25 days. The story goes that Kaasen,  exhausted, frostbite in his fingers, mustered the strength to stagger  off the sled and walk up to Balto.

Whereupon he collapsed and called  him a, quote, “damn fine dog.” But Balto was far from the only  damn fine dog in this relay. And Kaasen was far from the only human. In the end, 20 mushers and 150 dogs worked  together to deliver the life-saving medicine.

Overall, Nome suffered five or six  recorded deaths from diphtheria. There may also have been additional, unrecorded  deaths in the neighboring Native villages. But the new antitoxin serum certainly  kept those numbers from being higher.

Eventually, the larger package of 1.1  million units arrived from the West Coast, and the diphtheria outbreak ended. Most of the mushers involved in the Serum  Run seemed to go back to their lives. A lot of them were locals, part of the Inuit  or other native groups like the Athabascan.

And mushing was a job, after all. But Kaasen and Seppala would go  on to tour the country as heroes, along with their lead dogs. At  least in popular culture, though, only one of those dogs seemed  to get all the accolades, including a statue in Central Park,  that book that inspired Blair Braverman, and of course, a fictional movie  where he got voiced by Kevin Bacon.

Some newspaper coverage even credited Balto  as being the lead dog through the entire race. Which makes you wonder how they  thought the entire race was run and if they ignored at least  18 other human mushers, too. Seppala expressed some frustration at this.

He felt that Togo, as the dog who led the team  over the most miles, deserved credit, too. And while he later seemed to  regret some of those statements, specifically those that may have encouraged  slights against the other mushers or dogs, the BaltoTogo argument still comes up, today. [Braverman] I think that the  Togo-Balto issue is not unique to dogs. We often like to choose a solitary  hero to represent something that   actually required a lot of teamwork to achieve.

And so if there's any lesson here, it should  be not that Balto doesn't deserve his glory, but that Togo deserves it too, and  so does every single participant. [MAYA] After about a year of touring the country, Kaasen returned to Nome, and  reluctantly left Balto and the   rest of the dogs with a circus owner and promoter. Is that not so crazy? Unfortunately, the dogs soon wound up in a  dime-museum sideshow in kind of poor conditions.

But in 1927, the public raised funds to buy Balto, and he lived the rest of his  life in the Cleveland Zoo. He did not get to have six  puppies with his love interest and watch one of them become  the matriarch of a wolf pack, and another join a team running  the mail to and from Nome. Because the real-life Balto was neutered.

Togo, meanwhile, was given to a breeder  to live out his days in comfort. He died in 1929, at the age of 16. Today, a significant percentage of modern racing   sled dogs can trace their  family tree back to Togo.

Including our final expert on  sled dogs: An actual sled dog. [BRAVERMAN] Flame! Come on! This is Togo's great-grand...  great-great-great-great-great-grandbaby, probably.

She's completed the Iditarod. Come on! Come on!

Stay, stay right there. Don't keep jumping. There we go.

Okay, this is Flame.  She's completed the Iditarod. She's done the longest dogsled races in the world. She's placed top five.

She's… …very distantly related to Togo. She likes being on my lap. She's a little nervous about  why she is talking to a camera. [MAYA] And in case you’re wondering, Flame is, in Blair Braverman’s  own words, not a lead dog. [BRAVERMAN] Flame is a team dog, so  she's just in the middle of the team.

And her other role on the team is that she  has never, ever, ever been dropped from a race because she's so clingy that she would not  tolerate being left behind at a checkpoint. And as a result, she has mushed thousands  of miles out of pure spite and jealousy because she would be too jealous if I  mushed with other dogs who weren't her. [♪ MUSIC] [MAYA] As we can see with Flame,  sled dogs are still very much around, though not in the numbers they once were. Their role in everyday life has largely  been supplanted by cars and snowmobiles.

But they’re still an important part of the culture  and honored in sled dog races that continue today. And thanks to an additional  century of artificial selection,   evolution has continued to act  on them in fascinating ways. [HUSON] So modern sled dog lineages, I think the big thing that's  different now is most modern sled dogs are being selected as a sporting  athlete, not necessarily a working dog. So they're still considered a working dog,  but the type of work has shifted a little bit.

So, the older sled dogs that we had, they were selected as sled dogs,   but in a certain environment and for  certain types of working conditions that were more of the larger,   the larger freight pulling dogs or maybe a  slightly smaller, very long endurance dogs. And then compared to our modern  dogs, it's all about speed. So it might be speed on top of  endurance because some of our   dogs are still running extreme endurance events that reflect the original Nome serum run.

But other sled dogs are doing much shorter  events that are anywhere from three or 30 miles. [MAYA] And the sport of dog sledding now extends  much farther as well, even to places without snow. [HUSON] The sport has actually really expanded,   with kind of like people’s  resources and what’s available. So there's actually a whole realm  of what they call dry land racing. So that means you're racing on dry land.

So you're racing on dirt, on  grass, in the mud sometimes. [MAYA] And research on them continues. [HUSON] For years, I've been studying  the genetics of different traits, like their speed, their  endurance. their heat tolerance, what makes them a good lead dog, and we've just gotten to the point where  we published some papers looking at our   ability to predict these traits on  sled dogs using their DNA sequence. So the idea behind it is: Could you take a puppy and get a DNA sample  from a puppy and be able to predict, well, this dog might make a great Iditarod dog versus   this dog might be better for mid  distance or maybe even sprint? Or it could be a lead dog or versus, you  know, it should be somewhere else in the team. [MAYA] Meanwhile, thanks to  readily available vaccines, diphtheria has all but disappeared  in many parts of the world.

The last recorded case of respiratory  diphtheria in the United States was in 1997, thanks to near universal childhood vaccination. [TRUELOVE] It's really one of the true  successes of medicine and public health that we have reduced the number of deaths  from massive amounts to negligible numbers, numbers where we don't even remember that this  is a disease and people don't even care about it. So we should celebrate that, right? But we should also remember that it is not gone.  We have not eliminated or eradicated this disease. [MAYA] Diphtheria still appears in places   where access to vaccinations or  antibiotics has been disrupted, like in refugee populations or in war zones.

And it can be just as deadly as it once was. A 2011 outbreak in Nigeria killed roughly  40% of the children under 4 who got infected. Plus, over the last few years, the  number of cases have been rising, thanks to disruptions to vaccination campaigns   caused by the Covid-19 pandemic  and vaccine misinformation.

According to the World Health Organization, there were over 25,000 reported cases  of diphtheria in 2024, globally. Which doesn’t include the  cases that went unreported. [TRUELOVE] I worry that if we continue  to see these reducing trajectories we might become at risk for these outbreaks again   in the US. Like we saw this,  we’ve seen this in the past.

This is what has happened in these outbreaks  in Venezuela and Yemen and in the Rohingya where they have this interruption  in their access to vaccination that ends up causing an outbreak to occur  because you know the kids don't get vaccinated. We saw this with the soviets when the  Soviet Union collapsed in the 1990s that there was this massive outbreak  throughout the former Soviet Union countries. [MAYA] Part of the work Dr.  Truelove and others are doing is figuring out how to respond if  and when large outbreaks do occur. [TRUELOVE] As I mentioned,  our diphtheria antitoxin   is currently still produced using horses. One of the problems with that is that we have a   global stockpile that does not meet  the needs of a large-scale outbreak.

How do we increase those numbers? But also, you know, some of the work  that we're doing now is really focused on how we better use those small numbers in  the cases where we actually need them. [MAYA] The next time we run  into another Nome situation, we definitely don’t want to rely on any last  minute dog sled relays to save everyone. Sled dogs are remarkable animals.

They’ve been our companions for at  least nine and a half thousand years. I’m sure history is full of stories worthy  of adapting into a kids movie or three. And like Hollywood’s human protagonists,  sled dogs don’t all have one look.

Some are your classic purebred husky,  and some are like our new friend Flame. And yes, some are such mutts that they  wind up being mistaken for a wolfdog. As different as all their  special double coats appear,   though, they all share some pretty  spectacular genetic adaptations.

And it’s thanks to those  adaptations that over 100 years ago, all those dogs and mushers were able to brave the   bitter cold and save a whole  town from a horrible disease. [♪ OUTRO]