Current global epidemic of obesity is mainly related to increased consumption of high energy density foods and sedentary lifestyle that leads to a positive energy balance with subsequent accumulation of fat stores, primarily in genetically predisposed individuals. However, additional pathogenetic factors should be considered, including an infection. Several viruses causing obesity have been described in mice, chicken, rats, hamsters and monkeys. In humans, a significant positive association between being overweight and IgG antibodies was found for Helicobacter pylori and Chlamydia pneumoniae. This association of bacterial infections with increased BMI might not represent a causal relationship but could be a marker for greater susceptibility of obese individuals to infection. Crucial role in the development of "infectious obesity" in humans may be played by adenovirus infection, particularly AD-36 type that is also capable of inducing obesity in experimental animals as chicken, mice and monkeys. AD-36-induced obesity is paradoxically associated with lower levels of serum cholesterol and triglycerides both in humans and in experimental animals. Moreover, AD-36 enhances insulin sensitivity and improves hepatic steatosis. AD-36 effects in target organs as adipose tissue, liver and skeletal muscle are mediated through the viral protein E4orf1. This way AD-36 improves metabolic profile, as indicated by a greater glucose uptake by adipose tissue and skeletal muscle, reduced glucose output by hepatocytes, increased adiponectin levels and increased expression of adipogenic genes as peroxisome proliferator-activated receptor gamma. If E4orf1 improves glycemic control without reducing dietary fat intake and body fat stores, this viral protein would be highly valuable to develop novel anti-diabetic agents that mimic its effects.Key words: obesity, infection, adenovirus AD-36, diabetes mellitus, lipid profile, insulin sensitivity.