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A prion.
A prion, or misfolded protein caused by genetic mutation.
prion
infectious proteins called prions
People do not get Mad Cow Disease. No human can get mad cow disease but humans can be infected by eating meat from a contaminated cow that has mad cow disease. The disease in people that has been associated with humans is called variant Creutzfeldt Jakob disease (vCJD) that is also a progressive fatal neurological disease.
There is no such thing as "cow disease" unless you are referring to MAD cow disease, which is something else entirely.
A cow that died at a dairy farm in California was randomly chosen to be tested by the USDA. After being tested, it was discovered that though the animal was asymptomatic (i.e. it showed no signs of the disease), it had BSE (also known as Mad Cow Disease). For more information, see the related link.
It's not sad cow disease, it's MAD cow disease. Its a brain disease that can cause irrational behavior in cows.
Prions can cause an infection. They are simple proteins that can replicate under the right circumstances and one of them causes Mad Cow disease.
Smallpox. Louis Pasteur discovered the vaccine for small pox, using the cow pox organisms.
We are aware of mad cow disease
Typically a cow with Johne's Disease (which is a disease where a cow has chronic diarrhea) is considered safe to be slaughtered for food.