The disease is as incurably fatal as it is ghastly: victims move jerkily at first, then decline into nightmarish dementia. Long known to affect sheep, it suddenly appeared in cattle. Now it is infecting people-- at least in Britain, where everyone who has ever eaten beef wonders if they’ll be the next to get the human form of “mad-cow disease.”

Even Hollywood could not have written a more labyrinthine, horrifying plot, which connects a fatal illness found among cannibals in Papua New Guinea; a heretical scientific theory about a previously unknown, nearly indestructible infectious agent; a food chain gone shockingly awry; a rare brain disease affecting old people that is suddenly showing up in young people and public officials in deep denial about the terrifying extent of the threat. It would make for diverting, if scary, theatrical fiction-- except that it’s true.

Interviewing scientists at the forefront of the research effort, NOVA puts together the most current picture on what’s happening and why. What scientists are finding is making them rethink many fundamental assumptions about epidemiology and may hold startling implications for the future of public health. Already two Nobel prizes have gone to American researchers unraveling different aspects of the mystery, including its possible origin in a microscopic bit of inanimate matter called a prion.

So far the human death toll has been small-- about twenty total-- and the United States seems to have been spared detectable infection. But the long incubation period of the human form of mad-cow disease-- up to several decades-- makes it impossible to say how widely it has already spread and how many will ultimately die.