Mad cow disease, first identified in 1986 in the United Kingdom, is caused by prions – misshapen proteins that can replicate once they get into an animal’s body.

How is mad cow disease spread?
Humans can acquire mad cow disease by eating tainted beef. Since prions can exist in the environment for years, it can also be transmitted by exposure to cow urine or feces, though this means of transmission isn’t as common.

Once infected, a person can also spread the disease to other humans via saliva, urine, feces, or sexual activity.