An excerpt from Penny S.'s link:

"The obesity explosion that has swept the Western world over the past 30 years may have been caused by a virus, scientists have said.

Researchers have discovered new evidence for an illness they have called "infectobesity" – obesity that is transmitted from person to person, much like an infection. The agent thought to be responsible is a strain of adenovirus, versions of which cause the common cold. It has already been labelled the "fat bug".

There are more than 50 strains of adenovirus known to infect humans but only one, adenovirus 36, has been linked with human obesity.

Now scientists at the University of California, San Diego, have found that children who showed evidence of infection with adenovirus 36 were more likely to be fat. In tests on 124 children aged eight to 18, the virus was present in more than 20 per cent of those who were obese, compared with less than 6 per cent of the rest. Among those infected with adenovirus 36, four out of five were obese..."