They fed ground up chickens to cows, then fed ground up cows to chickens, who went back to the cows = prions. Run the cycle on repeat until there's enough prions in the cows to burn holes in their brains, until the solid muscle matter is literally like a kitchen sponge = mad cow disease.

Prions are a super spicy protein from cannibalism. Unfortunately, any animal can make them, and prions are hard as fuck to kill, surviving a week exposed to the elements on a surface and will transfer species.

We call Mad Cow, Kuru in humans.

Cannibalism is baaaaad