The best defense is avoidance of danger; proper hygiene can prevent a pathogen from gaining that first toehold. Instinctively slapping at a mosquito is not just an attempt to spare oneself the minor annoyance of a mosquito bite. It may
also prevent a long list of serious insect-borne diseases, of which malaria is the best known. Is the itch of a mosquito bite just part of the insect’s nastiness? It may be merely an accidental result of the chemicals the mosquito uses to ensure that our blood flows freely, but it may also be our adaptation for avoiding future bites. Imagine what would happen to a person who did not mind being bitten by mosquitoes. And imagine how successful a mosquito could be if its biting were not noticeable!
Our tendencies to avoid contact with people who may be infec- tious may have the same significance. Likewise, an instinctive disgust motivates us to avoid feces, vomit, and other sources of contagion. Our tendency to defecate away from others may prevent the infec- tion of close associates, and social pressures to conform to such prac- tices may protect us from infection by others.