Prion diseases. In order to work properly, proteins have to be folded in a certain way. Misfolded proteins typically don’t work as they should. Prions are misfolded proteins that cause other healthy proteins to misfold when they come into contact with each other. This causes all sorts of medical issues.
Mad Cow Disease is one of the more infamous prion diseases. In humans, it manifests as Creutzfeldt-Jakob disease. It basically causes your brain to melt, as the proteins holding it together all get misfolded. It has a 100% fatality rate, typically within 1 year of initial diagnosis. Fatal Insomnia is another prion disease, where the area of the brain that controls sleep is affected, but the body’s need for sleep still persists. You just become totally unable to fall asleep, until you fucking die from sleep deprivation.
The big issue with prion diseases is that they’re totally 100% incurable and untreatable. Once you have come into contact with the prion, you have a death timer. Prions are also extremely resilient. They aren’t destroyed by time, decomposition, fire, or even caustic chemicals. So they’ll just sit there, waiting for someone to come into contact with them.