WEIGHT gain may be infectious. Evidence is growing that many people who are overweight may have a virus to blame.

Some have long thought that a virus is involved in some cases of obesity, but the idea is still highly controversial. Now a study shows for the first time that this virus, called adenovirus-36, is more often found in people who are obese than in those of a healthier weight . We don’t know yet how people catch the virus, but it may spread from person to person. …