 Bovine Spongiform Encephalopathy (B.S.E.), nicknamed "Mad Cow Disease" was as unanticipated as AIDS, and is arguably as insidious. It arose from centuries of relative obscurity in sheep to sweep through British cattle herds with staggering speed. It is caused by an infectious agent whose precise identity is still in dispute. It seems to be unusually adept at breaking through the biochemical barriers that usually make it impossible for disease-causing organisms to jump from one species to another. It has already caused the deaths of at least 20 humans, most of them young. And it may be incubating within an unknown number of people who have consumed British beef products over the last decade. These gruesome facts alone make the story worth telling.