Think it's bad diet and lack of exercise that's making you gain those extra kilos? You might be wrong, as a new study has suggested that the real culprit could be some virus.
Researchers said that it could be due to a combination of issues, including viruses or something else that affects cells or organs, reports News.com.au.
Scientists noticed that laboratory rats and mice on strict diets had gained weight just as domestic pets and feral animals living around humans had.
To trace the reason, they looked at more than 20,000 animals from 24 populations of eight species living in or around industrialised societies.
"Surprisingly we find that over the past several decades, average mid-life body weights have risen among primates and rodents living in research colonies, as well as among feral rodents and domestic dogs and cats," they said.
In female mice they found an 11.8 per cent increase in body weight per decade from 1982, female cat weight had increased 13.6 per cent per decade, and male dogs had experienced a 2.2 per cent increase.
The scientists' concluded that apart from obvious explanations such as over-eating and lack of exercise, there are many others, including the accumulation of endocrine-disrupting chemicals, and that obesity is of infectious origin.
The findings were published in Proceedings of the Royal Society B.