Marital infidelity is usually examined solely in terms of strategies of men and women, with an emphasis on the enhanced payoff for male infidelity (provided he can get away with it). What are not clear are the strategies used, in terms of how often to engage in extra-marital affairs. It has been proposed that female strategies are governed by a “decision” to maximize the genetic diversity of her offspring, in order to better guarantee that at least some will survive against a common pathogen. This strategy would then impact on the strategies and diversity of pathogens. I make a number of predictions about both strategies and the genetic diversity of humans and pathogens, couched in game-theoretic terms. These predictions are then compared with the existing evidence on the strategies used by women and also in terms of the genetic diversity of human populations.