According to scientists at Louisiana's Pennington Center in the USA, a common virus could cause obesity.
(They took stem cells from fatty tissue taken from liposuction patients, and exposed some to adenovirus-36. After the cells had been left for a week, those exposed to the virus had changed into human fat cells, while those left to grow without the virus had not. So, now you know. I'm not fat, I've just got an infection.)