Being socially isolated — or more specifically, feeling that you're lonely — is strongly associated with decreased immune system function. This means that if you have a chronic health challenge, like herpes (which has been studied specifically), then feeling unsupported by your friendships can have a direct implication on how many outbreaks you have. Socializing may keep your immune system in good working order, by being called into action to help ward off disease. Interestingly enough, there might be a reverse relationship as well — with our immune systems affecting our social behavior.