Many things are contagious. Chicken pox, measles, and even exercise. Exercise? Yes, it's true. The concept of "exercise contagion" is a thing. Researchers wanted to know if exercise habits were contagious in a social sense. In other words, if you saw on Facebook that your friend ran three miles, would you be inspired to go out and run four miles? Mostly, yes, that is pretty much what happens.
Data from runners who checked into global social networks via apps or fitness trackers was analyzed, and while there are some factors that impacted exercise like weather or work obligations, overall, people are more likely to exercise if their friends are also working out. The reasons behind that are not entirely clear. Sometimes looking at the behavior of someone who is doing more can inspire others to do more. Conversely looking at the miles logged by those who are doing less, can spur more exercise as a way to maintain superiority. Whatever the specific motivation, it really does pay to hang around with people that exercise, if you want to up your fitness game. The effect was stronger in men than in women, but was definitely apparent in both sexes.