Fixation of adaptive immunity only takes place for a certain range of immune efficacies. The evolutionary process takes place at fixed values of innate and adaptive immune efficacies and the fraction of hosts with adaptive immunity is recorded in the stationary state. The. x-axis corresponds to the (externally fixed) efficacy of the innate immunity; the y-axis to the (externally fixed) efficacy of adaptive immunity; the color code indicates the fraction of hosts with adaptive immunity. Regions in red indicate fixation of adaptive immunity, while those in blue denote its loss. Coexistence of hosts with and without adaptive immunity takes place in regions with lighter colors. The evolutionary process takes place in (A) a well-mixed scenario, (B) a structured population without possibility of suicide, and (C, D) a structured population with the possibility of evolving suicide, with a maximum suicide efficacies of CD
= 0.3 (C) or CD
= 1 (D).