The anthrax letter attacks of 2001 drew a lot of attention to the question of biological warfare, sparking fears that terrorist-funded biologists could create "superbugs," bacteria or viruses designed to kill. But efforts to build designer bugs are not always malicious. In one unusual form of biological attack, researchers are engineering viruses to seek and destroy the cells that run amok in cancer patients. After more than 10 years of lab work, researchers in the field of "oncolytic therapy" have reached a sort of critical mass, deploying their designer viruses in a number of human trials.

The idea is astonishingly simple: let viruses do what they always do -- but only to cancer cells. All viruses infect host cells and trick them into replicating the virus until the cells burst, releasing the new viruses. But unlike other viruses, oncolytic, or cancer-bursting, viruses reproduce in and destroy only cancer cells, leaving normal cells pretty much alone. "Viruses are a parasite, and they want to grow in cells that replicate very effectively to have the highest opportunity for themselves to replicate," says Matt Coffey, chief scientific officer of Oncolytics Biotech in Calgary, Alberta. "Cancer cells fit that bill."