For Stanley B. Prusiner, M.D., vindication came in 1997 when he won the Nobel Prize in Physiology or Medicine for his discovery of the potentially infectious proteins he called prions. During the 20 years leading up to the honor, Prusiner was seen as a heretic for his view that “rogue” proteins could cause disease. “How is it that a protein can be infectious?” asked Prusiner, professor of neurology, virology and biochemistry at the University of California, San Francisco, during a visit to the medical school in November. The ubiquitous and normally harmless prions, Prusiner found, can take on an abnormal conformation and set off a chain reaction of malformed cells that trigger diseases of the brain, such as scrapie in sheep, mad cow disease and Creutzfeldt-Jakob disease. Prusiner is now looking for compounds that will thwart prions’ pathogenesis.