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Do people get fat because of their genes or because they eat too much? It鈥檚 an old debate, and one that has so far mainly ignored a third possibility: catching the wrong kind of cold.

In the late 1980s, Nikhil Dhurandhar, then a doctor in Bombay, India, heard that a chicken virus had the unusual side effect of making the birds obese. It belonged to a group called adenoviruses, which in humans are usually known for causing colds.

Dhurandhar investigated whether adenoviruses could make people fat. First he found that a human adenovirus called Ad-36 could make animals including chickens, mice and marmosets pile on the pounds. Then he found that 30 per cent of obese people had antibodies to Ad-36 鈥� suggesting that they had previously encountered the virus 鈥� compared with 4 per cent of lean people (快猫短视频, 5 August 2000, p 26).

This viral theory of obesity was initially ridiculed. 鈥淧eople dismissed it as nonsense,鈥� recalls Dhurandhar. But others have now started to replicate his findings. Eight other viruses have been linked with obesity in different animals, although no further ones have been implicated in people.

So how could a cold virus make you put on weight? Several possible mechanisms have been put forward. For one, the virus appears to slow a person鈥檚 metabolic rate. It has also been shown to suppress a hormone called leptin, which is known to put the brake on fat production. And Dhurandhar has found that when grown in the lab, human stem cells infected with Ad-36 tend to develop into fat cells.

Dhurandhar is now working with biotech firm Obetech, based in Richmond, Virginia, which has so far identified around 30 compounds that kill the virus. It also has a vaccine in animal testing. 鈥淧resumably the vaccine would be used in childhood but I can鈥檛 say if it would be one shot for life,鈥� says Obetech president Richard Atkinson.

Until then? Try not to breathe in when the person next to you sneezes.

Next: Diabetes

Read more: Six diseases you never knew you could catch