I just ate a cow brain taco, and it wasn't bad. Do I need to worry about getting mad cow disease now?

Not likely. Unless you live in a country in which bovines with mad cow disease (bovine spongiform encephalitis) have not been adequately eliminated from the population, your risk is slight.

Related Questions

I've heard that the brain, spinal cord and other central nervous system tissue is where mad cow disease is found. Would steak be ok to eat?

Well... muscles cont. Contain nerves. I would not eat any part of the cow that had mad cow. But... Eating the brain and spinal cord or tail (which contains the spinal cord) would obviously be worse than a slice of steak.