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 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.