Here's a news report suggesting that obesity may be "contagious". The research is not focused on obesity being contagious like a virus, but as in "socially contagious". The research suggests that obesity may be spreading due to social norms.
Specifically, the research contends that having overweight friends increases the risk of being overweight yourself. I think this whole line of research is very believable. These type of "network effects" exist for lots of other social conditions, so why would obesity be any exception? I think this also shows that obesity can''t be treated in isolation - the effects of the physical and social environments must be taken into account.