Obesity is increasing rapidly in the United States as well as in other countries. The World Health Organization considers obesity a worldwide epidemic that poses a major public health threat. In humans, obesity causes or exacerbates a, number of other diseases and co-morbidities. Etiology of obesity includes genetic, metabolic, social, behavioral and cultural factors. Although obesity has multiple causes, an often overlooked possibility is that of obesity due to an infection. Over the past two decades, seven pathogens are reported to cause obesity in animals. Canine distemper virus was the first reported obesity-promoting virus. Rous-associated virus-7, an avian retrovirus, causes stunted growth, obesity and hyperlipidemia in chickens. Borna disease virus was noted to cause obesity in rats. Scrapie agents were reported to induce obesity in mice. These pathogens appear to produce obesity by damaging the central nervous system. Next, three adenoviruses were reported to promote obesity, but their mechanisms are not clear. Animals experimentally infected with SMAM-1, an avian adenovirus, or two human adenoviruses, adenovirus type 36 (Ad-36) and Ad-37, developed adiposity. Notably, SMAM-1 and Ad-36 were associated with obesity in humans. Although more research is needed to further define the mechanisms and the role of pathogens in the etiology of obesity, they should be included in the long list of potential etiological factors for obesity. Determination of the role of pathogens in human obesity is critical for its successful treatment and prevention.