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.