In my opinion, the researches have identified a correlation without proving causality. The truth is, people who are chronically bitter and depressed never want to leave home, and therefore never get exposed to runny-nosed kids and their snot-swabbing parents, and therefore never carry home a billion infectious germs on the tips of their fingers for later transplantation into virgin, if depressed, mucous membranes. And therefore they have no experience fighting off other people’s virulent diseases, and are much more prone to develop infection when finally exposed.
Of course, the researchers didn’t bother to ask me. Maybe that’s because I never leave home.