In this scientific mystery, NOVA ventures to the front lines of medical research where scientists are scrambling to understand the strange new ailment popularly known as "mad cow disease." Highly infectious and incurable, this disease has claimed the lives of nearly a million cattle in Britain, and a variant is responsible for a handful of deaths in humans. Millions more people may have been exposed, and now the race is on to determine if we are on the brink of another deadly epidemic like AIDS or Ebola. What scientists are finding is making them rethink many fundamental assumptions about epidemiology and may hold startling implications for public health in the future.