Obesity is associated with adenovirus 36, which is found in 30% of obese people, but only in 11% of non-obese people. It has further been demonstrated that animals experimentally infected with adenovirus 36 (or adenovirus 5, or adenovirus 37) will develop increased obesity. Adenovirus 36 induces obesity by infecting fat cells (adipocytes), wherein the expression of the adenovirus E4orf1 gene turns on both the cell's fat producing enzymes and also instigates the generation of new fat cells. Evidence suggests that obesity can be a viral disease, and that the worldwide obesity epidemic that began in the 1980s may be in part due to viral infection.Obesity is also associated with higher gut levels of certain Firmicutes bacteria in relation to Bacteroidetes bacteria. Overweight individuals tend have more Firmicutes bacteria (such as Clostridium, Staphylococcus, Streptococcus, and Helicobacter pylori) in their gut, whereas normal weight individuals tend have more Bacteroidetes bacteria.
