I've written before about the extremely fascinating link between the immune and nervous system, hypothesizing about such interactions having a role in "sickness behavior". Such behavior may cause someone ill to not feel up to participating in social interaction thus delimiting spread of a potential contagious disease.
There is no doubt that social interaction fosters the spread of infection. In fact, the rise of large settlements (proto-cities) changed the infectious milieu our ancestors faced as the transitioned away from a nomadic hunter-gatherer lifestyles. In cities, social interaction is the norm and it presents a constant challenge to the immune system as it faces a heightened exposure to microbes from other humans, some of which can be deleterious if allowed to proliferate.
So if social interaction is unavoidable, how does one protect oneself. Vaccinations, hand hygiene, social distancing, and cough etiquette are all fairly recent developments. Before these technologies, humans had to cope on their own and those evolutionarily endowed with a coping mechanism would be favored.
New research published in Nature, helps flesh out this "inflammatory reflex" a little more. A group at the University of Virginia has discovered that the molecule interferon gamma -- a well characterized means organisms use to fend off pathogens -- is intimately linked with social behavior. To oversimplify, when one is in a social context interferon gamma increases so as to protect one from the microbes one will face from others. When the molecule is blocked in mice, social interaction decreases.
The implications of this research are far reaching and may lead to further investigation of how interferon gamma may influence pathological social deficits.