The Selfish Gene

by Richard Dawkins

normally harmless or beneficial bacteria can turn nasty, even causing lethal sepsis, in a person who is injured. A doctor might say that the person’s ‘natural resistance’ is lowered by the injury. But perhaps the real reason is to do with games of Prisoner’s Dilemma. Do the bacteria, perhaps, have something to gain, but usually keep themselves in check? In the game between human and bacteria, the ‘shadow of the future’ is normally long since a typical human can be expected to live for years from any given starting-point. A seriously wounded human, on the other hand, may present a potentially much shorter shadow of the future to his bacterial guests. The ‘temptation to defect’ correspondingly starts to look like a more attractive option than the ‘reward for mutual cooperation’. Needless to say, there is no suggestion that the bacteria work all this out in their nasty little heads! Selection on generations of bacteria has presumably built into them an unconscious rule of thumb which works by purely biochemical means.