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Hopefully you've already eaten so this little flashback to the early 2000s doesn't put you off your lunch: The USDA has confirmed a case of mad cow disease in Central California, but in the same breath the agency said the disease didn't enter the food supply.

Still, just the rumor of such a gross disease was enough to "roil the markets," as Reuters put it. In fact, the rumor's probably worse than the reality, as USDA chief veterinarian John Clifford told reporters at a Washington, D.C. press conference the actual animal, which was found at a rendering facility, didn't enter the food chain, and its carcass would be destroyed, according to Bloomberg. It's the fourth case of mad cow disease to be confirmed in the United States.

So what is mad cow disease anyway? You may have forgotten since 2003, when the first case hit the United States after an outbreak in Europe. But there's a reason this stuff sets the markets to roiling: It's nasty. Known as bovine spongiform encephalopathies, it basically causes wasting in the brain that leads to madness and death. It's caused by a misshapen protein, called a Prion, The New York Times explained in a 2003 feature on Dr. Stanley Prusiner, who identified the disease in 1988. Per The Times' Sandra Blakeslee:

Prions (pronounced PREE-ons), he and others went on to establish, are proteins that as a matter of course can misfold -- that is, fold themselves into alternative shapes that have lethal properties -- and cause a runaway reaction in nervous tissue. As more misfolded proteins accumulate, they kill nerve cells. Animals that eat infected tissues can contract the disease, setting off an epidemic as animals eat each other via rendered meats. But misfolded proteins can also arise spontaneously in cattle and other animals, Dr. Prusiner said. It is not known whether meat from animals with that form of the disease could pass the disease to humans, he said, but it is a risk that greatly worries him.

The scary part is the notion that this disease could be transmitted by eating something that has it. But Ann Venemen, the secretary of agriculture in 2003, assured Americans that the transmission was mostly through offal, not standard meat. "One important thing to remember is that muscle cuts of meat have almost no risk," she told CNN at the time.