Zombies. You'd think we'd be done with them by now. But there are still zombie books and zombie comic books and zombie movies and zombie TV shows and zombie parades. Can we move on to like harpies or manticores or something?

(0:18) We can't seem to let zombies go and there are a bunch of theories as to why. What makes most sense to me is that disease remains the final serious predator of mankind, and dead people are a pretty serious reminder and traditionally a serious cause of disease. So yeah, dead people are scary, and they're scarier when they start moving around.
 
(0:35) Of course, there's also the whole brain-eating thing. Which leads me to prions. 

(0:38) Not prions, which are spelled the same but pronounced differently and are birds and also not preons, which are pronounced the same but spelled differently and are theoretical point particles that quarks may be composed of.

(0:49) But prions, evil little misfolded proteins that destroy brains and get passed from animal to animal, mostly through the consumption of nervous tissue, by which I mean brain eating. Proteins have a linear sequence determined by our DNA, but that sequence doesn't actually determine how the protein works. Getting proteins to work relies on getting them to take the correct 3 dimensional shape, and how this happens is super complicated. But in prions, they are the wrong shape, and it's a bad shape.

(1:16) All prion diseases are caused by the misfolding of the same protein called PrP, which after years of study we have determined does... something.

(1:23) It seems to affect the formation of memories, and it's also apparently important to sleep, since a mutation of the protein in mice was found to cause "Fatal Insomnia". Yes, they die of not sleeping.

(1:34) But whatever this protein does when it's folded correctly, we know it's important, because it's encoded by what we call a "highly conserved sequence", a sequence in our DNA that's very similar across a broad swath of life. In this case: mammals.

(1:46) So all mammals have very similar forms of this "healthy" PrP protein. When we see highly conserved sequences, we know that even slight mutations to those areas are deadly. This is also why prion diseases are so easily transferred from mammal to mammal, because the proteins are very similar.

(2:00) But the real problem here isn't that the incorrectly folded proteins are dangerous, but that they're also really good at convincing healthy proteins to change shape to the dangerous form.

(2:09) As prions convert more and more healthy proteins into prions like themselves, they clump together and accumulate in brain tissue. These clumps of prions kill the brain cells, causing holes in the brain, which leads to the brain tissue getting, like, literally spongy, which is why the technical term for Mad Cow Disease is Bovine Spongiform Encephalopathy, which, literally translated means "Cow Spongy-Brain Disease."

(2:30) I like how sometimes when you translate a Latin name into an English one it makes scientists sound like four year olds.

(2:34) Spongy brain-tissue diseases, it turns out, make you super weird and, eventually, dead. Symptoms of these diseases include: unsteady gait, deterioration of speech, increased jerkiness of movement, and as the disease progresses, bouts of uncontrollable laughter, urinary and fecal incontinence, you know, the normal zombie stuff. There are, by the way, no cures for prion diseases. You zombify, and then you die.

(2:58) You might be wondering: we humans aren't super accustomed to eating brains or spinal cords or anything, so who even gets a prion disease in the first place?

(3:05) Well, sometimes, very rarely, the proteins can misfold in the brain all by themselves and sometimes people get stuff like Creutzfeldt-Jakob Disease because they've gotten a blood transfusion with contaminated blood or had surgery where the surgical implements weren't properly sterilized.
 
(3:17) But usually people get CJD from eating meat from infected animals, which is why they killed all those cows in the U.K. back in the day and why we don't, you know, feed cow brains to cows anymore.
 
(3:27) Back to brain eating though. We must examine the bizarre case of Kuru, a prion disease found in the Fore people of Papua New Guinea. Until the 1950's, the Fores had a custom of consuming their dead at mortuary feasts. You know, like you do, the men of the tribe customarily ate the muscles of their deceased friend, while women and children ate the brains. 
 
(3:44) Between 1920 and the 1960's, an epidemic of Kuru swept through the Fore people, killing upwards of 200 people a year. Women and children, of course, being the most vulnerable because they ate the most brains.
 
(3:57) But here's something fascinating. The scientists studying the Fores' Kuru epidemic found that some of the women who had participated in the mortuary feasts had a genetic mutation that protected them from experiencing the symptoms of Kuru. They had a resistance to the disease.
 
(4:07) Researchers studying this genetic resistance believe that this could mean that human ancestors could have been cannibalizing each other as long as 500 thousand years ago, which is probably the reason that prion diseases aren't more common. So, we do have cannibalism to thank for at least that one thing.
 
(4:23) Other good news, when someone dies of a prion disease in 100% of cases, so far, they have remained dead.
 
(4:30) Thank you for watching this Sci Show Dose. If you have any comments or questions or ideas, we're on Facebook and Twitter and of course down in the comments below. And if you want to keep getting smarter with us, you can go to youtube.com/scishow and subscribe.