It’s possible to “catch the fat” as one cause of obesity is a contagious human virus. While viral-induced obesity is not the only way people become obese, it is apparently a widespread and significant cause.
The human virus Ad-36, a member of the adenovirus [1] family, spreads via airborne means and infects the lungs first, much like the common cold virus. Then it spreads to the rest of the body. It has symptoms similar to the cold, such as sore throats. Worse, it apparently forces fat cells to multiply throughout the body, leading to an estimated 3 months of weight gain before the body mounts an effective defense to the virus.
One human study showed that about 1/3 of obese people have been infected with the virus versus 11% of thin people. Previous tests in mice and chickens indicated the Ad-36 virus can cause weight gain in those species.
Researcher Nikhil Dhurandhar has been responsible for many of the discoveries involving obesity-causing viruses. In India, he investigated how chickens infected with a deadly avian adenovirus became fatter, not thinner, before their deaths. When he moved to the US, he was prohibited from bringing along samples of the chicken virus. So he looked for human adenovirus strains that might have similar obesity-inducing effects, leading to the discovery of the Ad-36 link to obesity.
Further reading
Obesity ‘Virus’ Spreads Like Common Cold, Scientists Say [2]
Common Virus Boosts Fat-Cell Production — and Makes Fat Cells Fatter [4]
Wikipedia: Adenovirus Infection [5]
About Obesity [6]