I have been following this line of thinking for several years now - it was presented several years ago at the Experimental Biology conference and continues to be studied.
Scientific American reports,"New study results bolster the controversial hypothesis that certain cases of obesity are contagious. Over the last 20 years, some research has suggested that certain strains of human and avian adenoviruses--responsible for ailments ranging from the chest colds to pink eye - actually make individuals build up more fat cells. Having antibodies to one strain in particular, so-called Ad-36, proved to correlate with the heaviest obese people, and in one study, pairs of twins differed in heft depending on exposure to that virus. Now researchers have identified another strain of adenovirus that makes chickens plump."
Read more about this interesting work at Scientific American's web site.