Even if you’re not big on Alaskan history, you might have heard of the Iditarod sled dog race.

Every year since 1973, people have raced their dog teams across the 1500-kilometer stretch of Arctic tundra between Anchorage and Nome, Alaska. The race starts on the first Saturday in March, and to win, participants have to charge through some of the harshest conditions in the northern hemisphere.

But it’s not an exercise in torture. It’s inspired by what’s sometimes called “The Great Race of Mercy”, where dog sled teams raced along much of the same trail to deliver an antitoxin that would save thousands of people from a deadly diphtheria outbreak in the winter of 1925. In the early 1900s, most of Alaska’s remote towns were pretty much inaccessible during winter.

Train tracks connected the largest cities, but the most reliable way to travel between most places was by dog sled. Driven by so-called ‘mushers’ and pulled by teams of dogs, these sleds were used to transport people, goods, and mail during the long winters when the severe weather knocked boats and planes out of commission. In January 1925, that dog sled transportation network became one town’s last hope.

In December of 1924, Curtis Welch, the only doctor in Nome, Alaska, was treating an outbreak of sore throats and coughs in the town’s children, which he initially diagnosed as tonsillitis — a simple viral infection. When two of his patients died, he realized he was facing something much more serious: diphtheria, an extremely contagious disease caused by the bacterium Corynebacterium diphtheriea. It spreads through tiny droplets of mucus coughed or sneezed out by infected patients, which linger in the air and on surfaces.

At first, the disease looks a lot like tonsillitis, or even a common cold. But it’s way more deadly, because the bacteria invade the patient’s upper respiratory tract and produce a type of cell-slaying protein known as an exotoxin. Once it makes it into host cells, the exotoxin protein keeps them from manufacturing their own proteins, which rapidly kills them.

As dead and dying cells slough off the victim’s nose and throat and mix with the growing bacterial colonies, they form these thick, leathery grey coatings called pseudomembranes. They can make it hard to swallow or even breathe. And without treatment, the disease kills more than half of its victims.

But even back in 1924, it could be treated. Scientists would inject diphtheria into animals that were less susceptible to the disease, then collect the antibodies they produced in response and use them to make an antitoxin serum. When it’s treated with the antitoxin, the death toll from diphtheria drops to one in ten.

Unfortunately for the residents of Nome, the town’s supply of antitoxin had expired, and the vaccine, developed in 1921, hadn’t been administered widely. To keep the outbreak from getting even worse, Welch put the entire town under quarantine to reduce the spread between families and keep the disease from reaching the roughly 10,000 people in the rest of the region. But without antitoxin, thousands of people were at risk of infection — and death.

A national search found a suitable supply of the antitoxin in Anchorage, Alaska, but no one was sure how to get it to Nome. The closest trains brought it to Nenana, which was still more than a thousand kilometers away. Planes back then were open-cockpit, and with the worst weather in 20 years, they weren’t an option.

And sea ice blocked the shipping route. So the governor of the Alaskan Territory turned to 20 of the best mail-carrying dog sled teams to take the precious serum on the last leg of the journey. Over five days, 20 mushers and 150 dogs carried the 9-kilogram package of antitoxin between relay points, fighting white-out blizzard conditions and temperatures of -45 degrees.

Celsius across a trail that normally took three to four weeks to complete. Four of the animals died in the process. The teams averaged about 48 kilometers each, although some went much farther than others.

Leonhard Seppala and his lead dog Togo traveled 146 kilometers over the tundra and crossed the icy surface of the frozen Norton Sound. The last team, lead by musher Gunnar Kaasen and his lead dog Balto, made the trek to Nome in a whiteout so bad that Kaasen said he couldn’t see any of his dogs. He relied on Balto’s sense of smell to guide them.

They reached Nome in the early morning of February 2nd, just five days and seven hours after the serum was picked up by the first musher. Thanks to the rapid delivery of the antitoxin, only five of Nome’s 1,400 residents died during the outbreak. The national publicity of Nome’s dire situation fueled a campaign by health officials to widely vaccinate against diphtheria in the United States.

And that continues today with the TDaP vaccine. In the 1920s, there were 100,000-200,000 cases and 13,000-15,000 deaths from diphtheria every year. But the U.

S. has only recorded five cases in the last decade. And the state of Alaska still runs a yearly immunization drive alongside the commemorative. Iditarod sled dog race.

So in a way, those 20 teams saved an uncountable number of lives. Thanks for watching this episode of SciShow, brought to you by our patrons on Patreon. If you like your science with historical sprinkles, you might like our episode on the Bone Wars—the feud that rocked paleontology.