The idea that obesity is contagious and can be spread like a virus1,2 was a brilliant analogy that provided a convenient rubric for people to understand that obesity could be transmitted to other individuals. In this issue of JAMA Pediatrics, Datar and Nicosia3 present an innovative natural experiment to study whether adolescent and parental obesity spreads in social networks. The ideal experiment to test whether multigenerational obesity is spread within a social network would be to randomize a group of nonobese parents with nonobese adolescent children to live in environments with a low or high proportion of families with obesity. This thought experiment would be difficulty to implement, but the natural experiment by Datar and Nicosia3 was a proxy for this format by studying 1519 military families who were assigned to relocate to counties with obesity rates that varied from 21% to 38%. Data on height and weight were collected via parental self-report, and Datar and Nicosia3 also measured height, weight, and body composition in a subset of 458 adolescents. They also collected objective and self-reported data on built environment, and had data on how long the families lived in the new community, and whether they lived on or off the military installation.