In the U.S., the prevalence of obesity increased by 30% from 1980 to 1990, and this increase appears to be continuing. Although obesity has multiple etiologies, an overlooked possibility is obesity of an infectious origin. Six pathogens are reported to cause obesity in animals. Canine distemper virus was the first virus reported to cause obesity in mice, followed by Rous-associated virus-7, an avian retrovirus, which has been shown to cause stunting, obesity and hyperlipidemia in chickens. Next, the obesity-promoting effect of Borna disease virus was demonstrated in rats. Scrapie agents were reported to induce obesity in mice and hamsters. The final two reports were of SMAM-1, an avian adenovirus, and Ad-36, a human adenovirus that caused obesity in animals. Additionally, an association with human obesity is the unique feature of SMAM-1 and Ad-36. Although the exact mechanism of pathogen-induced obesity is unclear, infection attributable to certain organisms should be included in the long list of potential etiological factors for obesity. In addition, the involvement of some pathogens in etiology of obesity suggests the possibility of a similar role for additional pathogens.