What does positive social mood actually do?
For starters, if you're in a population that has enjoyed an extended period of positive mood, outbreaks of disease are less likely to happen. If mood is positive and you personally hold a relatively higher place in the social hierarchy, odds are even higher that you'll avoid an outbreak of disease.
Alas, of course, if those circumstances apply to you in reverse … well, be especially mindful to stay current with your flu shot (etc).
Let's get started.