In a windowless underground computer lab in California, young men are busy cooking up viruses, spam and other plagues of the computer age. The companies that make their living fighting viruses aren't happy about what's going on in Ledin's classroom.

Ledin insists that his students mean no harm, and can't cause any because they work in the computer equivalent of biohazard suits: closed networks from which viruses can't escape. Rather, he's trying to teach students to think like hackers so they can devise antidotes. "Unlike biological viruses, computer viruses are written by a programmer. We want to get into the mindset: how do people learn how to do this?" says Ledin, who was born to Russian parents in Venezuela and trained as a biologist before coming to the United States and getting into computer science. "You can't really have a defense plan if you don't know what the other guy's offense is," says Lincoln Peters, a former Ledin student who now consults for a government defense agency.