Prisoner's Dilemma simulations. Graphical output from DILEMMA, a computer program simulating the Iterated Prisoner's Dilemma (IPD) with the ability to incorporate parasites into the classic model to determine how prevalent parasite would need to be within populations to help favour cooperation over cheating. A) In the traditional IPD, the strategy "always defect" quickly eliminates the purely cooperative strategy "always cooperate" from the population. B) When parasitism is introduced into the population, "always cooperate" becomes a dominant strategy when infection rates are greater than 50% in the population. C) Strategies that combine cooperation with retaliation, such as "tit for tat", are successful in a heterogeneous population of strategies in the IPD (strategies defined in Supplementary Table 2). D) In the presence of parasites, "always cooperate" becomes a viable and dominant strategy in a heterogeneous population at parasitism levels as low as 35%. Output was generated using populations of 10,000 individuals, which engaged in 25 interactions per generation, for 500 generations. Each graph depicts the mean of 100 independent runs of the DILEMMA program. Standard error (SE) bars are not shown because values are less than 4.92 × 10-3, with the exception of the data points at 0.5 parasitism (b), which have SEs of 0.479.