Our modern world is seriously interconnected.

We’re constantly spreading people,  food, and goods across continents. And as the early days of COVID showed  us, diseases spread along with that.

An outbreak in one place can lead  to an outbreak in another place,   which can lead to a pandemic in a  matter of months, or even weeks. But despite this interconnection, there are  a few diseases that are serious homebodies,   sticking to just one geographic area. So we are going to go to them.

Grab your passport and pack a bag,   because we’re going to visit the most  geographically isolated diseases on Earth. What could go wrong? Our first stop is in jolly old England, which  is home to variant Creutzfeldt-Jakob disease.

If you’ve ever heard of mad cow disease,   variant Creutzfeldt-Jakob disease  is the human equivalent of that. Note that this is different  from regular Creutzfeldt-Jakob,   even though the symptoms are similar. Both flavors of CJD are caused by misfolded  proteins called prions that clump up in the brain.

And these are troublemakers, because when  they find their correctly-folded counterparts,   they cause those proteins to misfold too,  which is how they build up over time. The difference between the two is how you managed  to get a messed-up protein in the first place. With classic CJD, researchers aren’t totally sure   how most of their patients  end up with the disease.

About 85% of cases are idiopathic,   meaning they just don’t know why  that first protein got messed up. The rest of classic CJD cases are usually genetic. But we know exactly how you get variant CJD,   because that is caused by eating meat  from those aforementioned mad cows.

More specifically, it happens when humans eat beef   from cows with a condition called  bovine spongiform encephalopathy. Eventually, that buildup of messed-up  proteins eats away at the tissue of   the brain, making it full of holes, like a sponge. This results in depression, anxiety, delusions,   and neurological symptoms like  dementia, and involuntary movements.

But the really scary part is  that once someone is infected,   they might not show symptoms for decades. And once they do, they’ll be  dead in a little over a year. There’s no cure for any prion disease,  and all of them are 100% fatal.

So that’s pretty bad. But the good news is that as  far as we know, it’s super rare. In the first twenty years after discovering  variant CJD, there were only 231 cases identified,   a little under 90% of which were in the British  Isles or directly across the channel in France.

Of course, it’s possible the number is higher, since it stays asymptomatic for so long.   It’s possible that a bunch of people have  been infected and we just don’t know it. So why are we not all constantly  freaking out about variant CJD? Well it’s very locked down, thanks  to governmental regulations.

When a cow is diagnosed with  bovine spongiform encephalopathy,   countries shut down the import of  beef from the place it came from. No infected beef, no variant CJD. So variant CJD is one British classic  that will probably never go truly global.

I will be keeping the fish and chips, though. Next, we’re going to head across the pond  to the home of Rocky Mountain Spotted Fever. Which is kind of a misnomer.

Because while RMSF was originally  discovered in the Rocky Mountains,   it’s not actually concentrated there. Instead, most cases are in the southeast  and south central United States. It’s a bacterial disease that spreads via ticks  infected with the bacterium Rickettsia rickettsii.

It starts with flu-like symptoms,  including fever and headache. Next comes the rash, which usually  starts off as flat, pink spots. Both of those is how we end up with the  “spotted” and “fever” parts of the name.

The good news is that once it’s diagnosed,  antibiotics can get it under control. The bad news is that it’s  kind of hard to diagnose,   because the symptoms are pretty non-specific. Like, a lot of diseases can  cause a fever and a rash.

And if Rocky Mountain Spotted Fever  isn’t diagnosed and treated quickly,   it can result in vascular damage,  paralysis, gangrene, and even death. There are three species of ticks  that can carry the bacteria,   so this particular disease is isolated to  the places where those ticks live – most   commonly North Carolina, Tennessee,  Missouri, Arkansas, and Oklahoma. But with climate change, there’s no guarantee that   those ticks will stay put,  so we may see cases spread.

In any case, the best way to avoid  it is to not get bitten by ticks,   which, yeah, easier said than done. Check yourself for ticks after you  spend time in nature, wear long pants,   and tuck your pant legs into tall socks. And yes, I know it looks dorky, but  that’s better than gangrene, okay?

The next leg of our trip takes  us to the American southwest,   home of stunning canyons, all  kinds of cacti, and…Valley Fever. Valley Fever, also called Coccidiomycosis,  is caused by a fungus found in soil here,   as well as parts of Mexico and South America. The fungus in question, Coccidioides, ends up  in the air after contaminated soil is disturbed.

Think a windstorm, or a construction  project, or even just animals digging. The tiny Coccidioides get inhaled, and then in  the warm, moist environment of the lungs, the   spores grow into spheres that burst and spread the  infection around the lungs and to other organs. It sounds gnarly, but in a lot of people, it isn’t  a huge deal, and they never have any symptoms.

Others might develop flu-like symptoms that  go away on their own in a few weeks or months. About 5-10% of people  develop serious lung problems,   and about 1% of people get what’s  called a disseminated disease,   where it spreads to other parts of the body,  like the brain, spinal cord, skin, or joints. On the bright side, it doesn’t spread   between people and treatment just  involves taking antifungal meds.

That said, since the fungus  spreads most effectively in hot,   dry conditions, climate change may  end up increasing its home range. We need to do more research  to confirm that, but in 2013,   Valley Fever was identified all the way  up in Washington State for the first time. And all these were people  who got it in Washington,   not people who traveled and were  diagnosed 'til they got home.

So Valley Fever may not be staying  in the valleys for very long. Okay, pack some snacks and a great book, because  we’re taking a loooong trip across the Pacific to   Australia, where they have a rabies-like  virus called Australian bat lyssavirus. Because of course Australia has its own rabies.

Like the name implies, Australian bat lyssavirus  is caused by a virus that’s very closely related   to our own rabies virus, and transmitted when  an infected bat bites or scratches a human. The symptoms of ABLV are similar to rabies too. They start off with flu-like symptoms, and  one to two weeks later end up with paralysis,   delirium, convulsions, and…death.

No one’s ever been cured of ABLV after  they’ve started showing symptoms. Though to be fair, there have  only ever been three cases,   so it’s not like they’ve had that  many chances to get it right. So far, the only thing you can do to  prevent it is to just… not touch wild bats.

Which is good life advice in general, but  especially when a fatal virus is in the mix. Bat bites and scratches can be so shallow that you  don’t even notice them when they happen, either. So even if you were around a bat and you don’t  think it touched you, it still totally could have.

If you do get bitten or scratched by an Australian  bat, public health officials recommend immediately   having the wound cleaned and getting the rabies  vaccine, which works against ABLV too… we think. In any case, it makes sense that  ABLV is only found in Australia,   given that people don’t usually  bring bats home as souvenirs. No infected bats, no disease spread,   so there’s very little chance that  ABLV will make it anywhere else.

There have been antibodies for a related strain  of lyssavirus found in bats in the Philippines,   but it seems likely that ABLV will  be staying in Australia for now. But we won’t be staying. We’re headed to Asia to meet Nipah,  another virus spread by bats.

Specifically, the little  cuties known as flying foxes. I know, they're adorable. But don’t touch them!

The first known Nipah outbreak was  in both Malaysia and Singapore,   and it shows up basically  yearly in Bangladesh and India. Infections happen through direct contact with  infected bats, or by eating fruit  that’s been munched on by a sick bat. But unlike any of the diseases  we’ve talked about so far,   you can catch Nipah from a person who’s infected.

It can pass from person to person via  body fluids, which requires close contact. Most of that transmission ends up being between  patients and their caregivers,  both at home and in clinics. The symptoms of Nipah are really all over the place.

Some people who get it are totally asymptomatic. Others experience respiratory  illness, like a cough. The unlucky ones end up with encephalitis,  or inflammation of the brain.

There isn’t any treatment other  than just trying to ease symptoms,   and between 40 and 75% of people who are  infected by Nipah virus end up dying from it. And that’s a big range because the  fatality rate of each outbreak varies,   depending on how much capacity an  area has to monitor for cases and   get patients into clinics once  they start showing symptoms. Because Nipah can spread from person-to-person, public health officials do worry about the   potential for it to spread  and cause a global pandemic.

But so far, standard infection control  practices like wearing appropriate PPE   when working with sick patients have  helped reduce transmission risk. So please, wash your fruit and wash your hands. Our next stop is in sub-Saharan Africa, home to  Ebola’s bigger, meaner cousin, Marburg virus.

Marburg virus is spread by the Egyptian  rousette bat, which lives in caves in,   well, Egypt, but in lots of other  places around the world, too. And I don’t know if you’re noticing a pattern  here, but maybe just, like… stay away from bats. Like a lot of diseases,   Marburg starts with flu-like symptoms  like fever, chills, and body aches.

Then comes a rash, followed by delirium,  bleeding, and multi-organ failure. There are a couple different strains of Marburg   virus that have different fatality  rates, anywhere from 22% to 90%. Like Nipah, it’s transmitted from  person-to-person via body fluids,   so any caregivers of sick patients are at risk.

Many outbreaks of Marburg have started among mine  workers working in bat-infested caves, and those   workers then inadvertently spread the disease  to their family members and healthcare staff. The good news is that most Marburg outbreaks  have only infected a handful of people each time. As of February 2024, there have only been  five outbreaks with more than 10 cases.

So it’s a nasty bugger, but as of  right now, we’ve been able to keep   it in check with public health measures and  really close monitoring. For now, anyway. The last leg of our tour takes  us north to see a little critter   who is vampiric in both name and behavior.

Meet Dracunculus medinensis,  also known as the Guinea Worm. It’s found in a few countries in Africa, mostly in  Chad, but also in South Sudan, Mali, and Cameroon. True to its scientific name, Guinea Worm  infection is a true horror story.

Their life cycle starts in ponds, where tiny  little water fleas swallow Guinea worm larvae. Those water fleas have to end up inside a host,  and cause an infection called Dracunculiasis. They get into that host either from people  drinking water from the infected ponds,   or from eating aquatic animals  that have ingested them.

But either way, the worm larvae end up in  the person’s digestive tract where they mate. And then the pregnant female begins to grow. And grow.

Until a year or so later, when  she’s two to three feet long. At that point, the mama worm migrates out  of the digestive system to just below the   host’s skin, where a painful blister forms. A day or three later, the  mother-to-be emerges from the blister.

And because this hurts, the host will probably  seek out water to relieve the burning pain of   the blister, which lets the worm release her  larvae into water, and the cycle begins again. There’s no treatment for guinea  worm disease besides removing   the worm after it comes out of the blister. [stammers] I just - I'm, like, very upset. [laughs] But even then, it takes weeks to remove the  worm, since you’ve got to make sure that its body   doesn’t break, in order to prevent a bacterial  infection from happening to that open wound. So you end up out of commission, in pain,   and unable to work while this worm who decided  to make you her home is sloooowly evicted.

But this nightmare has a happy ending. Or at least, it’s going to soon. In the mid-1980s, there were 3.5 million cases of   Guinea worm spread across  20 countries in Africa and Asia.

But thanks to the efforts of the World Health Organization, UNICEF, and The Carter Center, it has almost been completely eradicated. There were only fifteen cases of Guinea worm  disease reported in humans in 2021, in Chad,   Ethiopia, South Sudan, and Mali, and it’s on track  to be completely eradicated in the near future. If it is, it will be the first disease  to be eradicated without a vaccine.

Take a hike, Guinea worms! While all diseases are different,   there are a few patterns we see with  geographically isolated illnesses. For one thing, while all of them infect humans,  only a few of them spread from human to human,   which is part of what keeps the  diseases stuck in one place.

In a lot of cases, each person  who’s infected has to come in   direct contact with the disease’s primary host. And while people may be interconnected  and constantly on the move,   we’re less likely to bring infected  dust, ticks, or bats along with us. So you don’t have to let the fear of diseases  stand in the way of your travel plans.

Just pay attention to public health warnings, get  vaccinated against local diseases when you can,   and don’t forget to put your  liquids in a Ziplock baggie. And seriously. Stay away from bats.

If you liked learning about all of these  diseases, have we got the thing for you! Crash Course has just released a special Guest   Lecture documentary called  Consumption and Tuberculosis. This 45-minute documentary is  hosted by our friend John Green,   on-location at the Indiana Medical History Museum.

This is the story of the deadliest  infectious disease of all time. It’s a disease for which we’ve got a vaccine and  a cure, so why are so many people still dying? Tuberculosis is more than just a disease — it  reveals fundamental truths about who we are   as human beings, and how we have changed,  or failed to change, throughout history.

We’ll learn about tuberculosis from a historical,   cultural, and scientific perspective,  bringing us up to the present day. Check it out over on the Crash  Course page – the link is down below. And thanks for watching.