Mad cow's brain-wasting course inspires fear.(Knight Ridder Newspapers)

Byline: Faye Flam

PHILADELPHIA _ No longer can experts reassure us with absolute certainty that American beef won't expose people to the human version of mad cow disease, now that an infected cow has turned up here.

While odds remain remote _ only 153 people in Europe have contracted the disease since it appeared in 1995 _ it is not the numbers that terrify people but the harrowing trajectory of the disease, the way it eats away at the brain.

There is no way to prevent the onset, no test for exposure, no way to test blood, and no cure.

The so-called prions that cause mad cow disease are neither viruses nor bacteria but simply errant protein molecules. Prions survive cooking and digestion, and seem to hide out in the body for 10 years or longer before growing in pockets in the brain.

Some researchers have compared mad cow to the disease in Michael Crichton's "The Andromeda Strain," noting that fictional agent was a nonliving crystal. Prions can't multiply because they aren't alive, but once they get into the brain, they convert healthy proteins into their deadly shape. Once the disease takes hold, the pockets of bad prions expand and turn the brain into a spongy texture, which is why the cow version of the disease is called bovine spongiform ...