The epidemic swept the world. Fortunately, it was only the World of Warcraft, a popular online role-playing game. But that got the attention of real disease experts at Tufts and Rutgers Universities. That's because the accidental outbreak that attacked the virtual characters offered a unique opportunity to study how social groups can help spread a disease. In late 2005 the epidemic hit the World of Warcraft, played by millions. It all started with an error. One creature was supposed to infect only a few virtual players with so-called corrupted blood. But some of the nastiest virtual inhabitants exploited a flaw and spread the disease to unsuspecting masses. The virtual quarantines game designers tried to impose didn't work, in part because the virtual people didn't follow them, and so entire virtual cities were virtually destroyed. The experts were fascinated because they've have never had a way to realistically simulate how large groups of people will react to an epidemic. But the cure for a real epidemic still won't be as easy as the virtual one was—they just reprogrammed the computers.