inoculate people with viruses, of course. But in an analysis of 1,000 people, the scientists found that those with AD-36 antibodies a sign that they have been infected are significantly more likely to be overweight. 3 How could a virus make you fat? When fat cells are exposed to AD 36, they begin to multiply. "We are not saying all obesity is caused by a virus Genes, metabolism, and habits all play a role, the scientists acknowledge. "But at least one other possibility appears to be infection 4 As yet, no one knows how widespread AD-36 may be, or how big a role it might play in the current obesity epidemic. If the scientists' suspicions prove correct, the next step would be to develop a vaccine.