I have proposed that three things — a tendency to touch each other (e.g., shake hands), a tendency to touch near our mouths, and our tonsils — together form an early warning system for our immune system. The early warning system helps the immune system get tiny exposure to microbes circulating in the community. It performs self-vaccination. Like ordinary vaccination, exposure to tiny amounts of Microbe X protects against exposure to a large amount of Microbe X.
In Daniel Everett’s anthropological study of the Pirahã people (Don’t Sleep, There are Snakes: Life and Language in the Amazonian Jungle, 2009) he says the Pirahã “all touch one another frequently” (p. 85). “They loved to touch me too.” He has never seen kissing but “there is a word for it, so they must do it.” This supports the idea that a tendency to touch others is widespread.
If this theory is true, reducing microbe exposure to zero (e.g., sterile food) is a seriously bad thing. It’s been proposed that the polio epidemics of the first half of the 1900s were caused by cities becoming too clean.