Biologist Stanley Prusiner performed groundbreaking research on a new class of germ that slowly attacks the brain. Prusiner, who was born on this date in 1942, began researching prions in 1972 after one of his patients died of dementia resulting from the human equivalent of "mad cow" disease. Years later, he and his research team at the University of California at San Francisco discovered a new class of infectious agents -- which they called "prions" -- that are aberrant proteins. Certain prions, under unusual circumstances, make their way into brain cells. If the proteins that make up the prion take over the cell machinery, they can cause nerve cell destruction and disease. Originally met with skepticism, Prusiner's research won him the Nobel Prize in physiology or medicine in 1997 for his discovery of "a new biological principle of infection."

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