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Coming off of a global pandemic, you might be wondering what some of the deadliest diseases in human history have been. These top five deadliest diseases will have you reaching for the hand sanitizer and praying not to get sick. Join Hank Green for a spooky sickness episode of SciShow!

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 Introduction (00:00)

There's a lot of stuff that's out there trying to kill us right now, and today I'm going to talk about the ones that are the best at it. The five deadliest infectious diseases in the world. Now do yourself a favor; put away any food you might have nearby you and if you have a dog, you might want to move it into another room.


You might be tempted to say that the monstrous bastards I'm about to talk about right now are the deadliest organisms in the world, but that wouldn't really be true. Because all the diseases you're going to hear about today are caused by viruses, and, as you know, viruses are just protein-covered scumbags full of nucleic acids, so they're not generally considered living things. But still, you gotta hand it to them; viruses have probably been around for at least tens of millions of years and have managed to make a great living by ripping off our DNA like they're the freaking Pirate Bay, using it to copy themselves and pretty much master our asses for as long as there have been warts. But I'm not talking about warts here guys; it's much worse than that. I'm talking about the diseases with the highest known case fatality rates, which is how experts measure the deadliness of a disease. The percentage of people diagnosed with it who end up dying from it.

Remember the Spanish Flu? 1917 to 1918, killed like 30 million people worldwide, and basically changed the course of modern history? That was a strain of a virus called H1N1, and it had a case fatality rate of like 10 to 20%. The viruses I'm talking about are so much deadlier by comparison that Spanish Flu's basically not even worth calling in sick for.

 Nipah (01:35)

Take for instance, Nipah, which has an average case fatality rate of about 50%. It's named for a town in Malaysia where it was discovered in 1999 among pig farmers. Seems a bunch of them started coming down with severe respiratory problems, and inflammation of the brain that caused hallucinations, and seizures and not the good kinds. Wait, there's good kinds of seizures?

Outbreaks soon followed in India and Bangladesh. This time among people who had eaten fruit that was tainted by bats that carried the virus. The death rate in some of these outbreaks was 100%, and there are no treatments or vaccines for the virus. But what's really pants-poopingly terrifying about Nipah is that is soon proved to be easily transmissible among humans, no pigs, or bats, or pig-bats required. In 2001, there was an outbreak in the town of Siliguri in India, and 75% of those cases were traced back to people who had visited the local hospital; just by being in that building, they got it.

But hey, that doesn't affect you, right? Because odds are you're not a pig farmer and you're also probably not watching from your home in Siliguri.

 H5N1 (02:37)

Well, no doubt you've heard of H5N1. The virus formerly known as bird flu has been making the rounds mainly in Asia and Europe where it's often fatal to birds. Luckily it's rarely contracted by people, and it's not very good at jumping from person-to-person... until now.

You may remember a while back when I told you about how scientists genetically engineered bird flu to make it contagious among ferrets. And that's important because ferrets essentially have the same immune system as humans. Don't ask me why.

It's assumed that these new strains are contagious to us as well, which kind of sucks because the World Health Organization says that H5N1 kills at least 54% of the people who get it, usually from respiratory problems.

Now there is a vaccine for the strain that's out in the world right now, and, at least in the US it's been stockpiled by the government. It hasn't been made available to the public... yet.

As for the strains that were made to be contagious between mammals; they're currently kept under lock and key in labs in Wisconsin and the Netherlands. Hopefully big lock with thumb print and retinal scanner, voice activated. And some of the scientists who monkeyed with the virus have said that they've developed a vaccine, at least for the strains that they invented. So that's great. They can release it on the world and then sell the vaccine. That'll be great business for them.

Now Hank, you're saying, 'I'm not a Malaysian bat handler, and I've already stocked my pantry with enough Skittles and Diet Sierra Mist to get me through the bird flu pandemic. Tell me something that I don't know.'

 Marburg Hemorrhagic Fever (03:58)

All right, how about this? In 1967 German scientists from the town of Marburg were testing polio vaccines on monkeys imported from Uganda. You know you're gonna have a killer story on your hands when it starts with German scientists and lab monkeys.

Those scientists started coming down with some gut-wrenchingly, flop sweat-inducingly horrible symptoms: wicked fevers, diarrhea, vomiting, massive internal bleeding, until, for many of them, their circulatory systems just shut down.

It didn't take long for the scientists to figure out that there was a correlation between messing with monkey parts and contracting the disease, so they basically started studying themselves and their sickness, and isolated the virus that is today known as Marburg Hemorrhagic Fever.

That first outbreak in 1967 killed 23% of the scientists and, in some countries, including the United States, that's still the official case fatality rate. But the thing is Marburg keeps showing up, mainly in central Africa, and when it does, it kills more than 80% of its victims in outbreaks affecting hundreds of people.

Unlike Nipah, though, which can be transmitted through the air, Marburg can only be contracted by coming into contact with an infected person's body fluids or tissues, which are all over the place when they're actually infected with them. It's not something you generally think of. It's like, 'How would I get that person's poop on me?' Oh, it's because his butt is exploding!

We should all be thanking our gods for that because otherwise it would be lights out. The World Health Organization says that Marburg is one of the most virulent pathogens to ever infect humans, and national security types put Marburg on the top of the list of viruses that you do not want crazy people getting their hands on.

 ZEBOV (05:29)

And when those German scientists first isolated the Marburg virus, they realized that they were dealing with a whole new class of scumbag. A virus that has a completely different shape and attacked the human circulatory system. What they had discovered was the virus family, Filoviridae. This unwholesome lot includes a cousin of Marburg that is the second most fatal disease on our list, Zaire ebolavirus or ZEBOV.

As you can tell from its name, ZEBOV is just one of five species of the Ebola virus, and is the one that is responsible for the most outbreaks and is by far the deadliest.

Like Marburg, ZEBOV causes a wide range of flu-like symptoms like - I'm going to do this smiling - vomiting, and fever, and then moves on to failure of blood vessels, causing bleeding under the skin from mucous membranes. Is it better when I'm smiling? But unlike Marburg, ZEBOV has an average fatality rate of 83%. And in outbreaks in the Republic of Congo in the early 2000s, it killed more than 90% of the people it infected.

So how could you possibly get worse than that? What disease is deadlier than 90% fatal? And in what forsaken corner of the globe does it thrive? What I'm going to tell you, and it may be a little bit surprising, but first you might want to put your dog in another room.

 Rabies (06:35)

The deadliest disease in the world, the one with the highest, almost always worst case scenario, nearly perfect batting average fatality rate... rabies.

I know you're like, 'Huh? You mean, my adorable golden retriever over here is a vector for the greatest plague known to humanity? Isn't rabies like everywhere, and isn't there a vaccine?' Indeed, all of that is true, but the fact is that rabies has a case fatality rate of about 100%.

Once you've been diagnosed with symptoms of the disease, it means almost certain death. There've been fewer than ten recorded cases of people exhibiting symptoms of rabies and then living to tell about it. Less than ten.

So why hasn't the rabies virus brought humanity to its knees? Three reasons:

One: unlike all the other filthy scumbags, I've talked about today, there's a vaccine that's widely available for the rabies virus. Louis Pasteur developed it in the 1880's using tissue of dead, infected rabbits. And since then, the ball has pretty much been in our court. But it's important to remember that a vaccine is not a cure; it's just a preventative. Which brings us to reason two: 

Rabies has a really long incubation period. That's the time between when you're infected and when your symptoms start to appear. Marburg and Ebola have incubation periods of just a few days, but for rabies, it's like two to three months. That means you have almost 12 weeks to get vaccinated for rabies, even after you've been infected. And the vaccine will prevent the onset of symptoms. So you're basically taking the preventative treatment even after the virus is in your body, but you're not quite infected yet.

And the third reason we're beating rabies is education. At least in developed countries, public health education has been so good for so long that people get vaccinated as soon as they're bitten by an animal. And in many places, vaccinating pets is required by law. It's a sterling public health victory.

But rabies remains a serious problem in developing countries where both public health education and access to health care are scarce. And it is a terrible way to go. Early symptoms are flu-like, but after a couple of days, the virus targets the central nervous system. And then the victims becomes agitated, and delirious, and often begin to experience seizures. Paralysis strikes, mostly the throat and the jaw, making it difficult to swallow, which is why patients avoid water, giving it the nickname hydrophobia. Soon pulse and blood pressure begin to vary wildly, often leading to coma and eventually heart failure. So there's that.

 Conclusions and Credits (08:52)

Nipah, H5N1, Marburg, ZEBOV, rabies, but what disease execrable conditions have in common? Well, one thing you may have noticed is that all of these diseases are zoonotic, meaning that they are transmitted to humans from animals, or originally were anyway. With the exception of bird flu, they're all easily transmitted to us from other mammals, bats especially. I don't know about you, but my plans of adopting a pet bat are totally off the table now.

And what makes them transmissible? Well, all of these viruses belong to this order of viruses [Mononegavirales is displayed on screen]. You can try to pronounce it; I'm not going to. They're bullet-shaped virus with a single-strand of RNA inside. They're very good at connecting with animal and plant cells, and because they lack the enzyme that proofreads its RNA after it's copied, it mutates really easily and really quickly.

For these and other reasons, these viruses are responsible for many, many, many of the diseases that affect people like mumps, and measles, and all kinds of flu.

All right, now assuming that your dog is properly vaccinated, you can let it back in the room and give it a big kiss now, if you still feel like it. Sorry if I ruined your day, but someone had to do it. Aren't you glad it was me?

Thanks for watching this infusion here on SciShow. If you want more, go to and subscribe. If you have questions or ideas for future episodes, things you'd like us to cover, please leave them in the comments or on Facebook or Twitter. Goodbye.