While some researchers search for the roots of obesity in our genes, othersinvestigate infections. Scientists have known for two decades that, surprisingly,certain viruses can cause birds or mammals to become fat. Now scientists thinkthey have tracked down a bug that can cause obesity in people.2 The alleged culprit is human adenovirus-36, or AD-36. A study publishedin 2002 showed that AD-36 injected into monkeys causes dramatic weight gain.In some animals, the injected virus caused body fat to triple. It’s unethical to inoculate people with viruses, of course. But in an analysis of 1,000 people, thescientists found that those with AD-36 antibodies—a sign that they have beeninfected—are significantly more likely to be overweight.