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.
