This invention relates to the relationship between infection with adipogenic adenoviruses, such as adenovirus-36 (Ad-36) and the etiologies of obesity and obesity-related cancers and other diseases. Additionally, the invention relates to assaying a subject for Ad-36 status and using Ad-36 status as a marker for the presence of cancer, to predict the future risk of developing cancers, such as breast or prostate cancer, and/or to predict the aggressiveness and prognosis of these cancers.