Synchronous parasites frequently transmit better from single infections. Smoothed relative fitness (ratio of cumulative probability transmission for synchronous:asynchronous strain) with increasing competition among merozoites (x axis) and investment in producing transmission stages (y axis). Synchronous infections transmit better in the white/gray areas. Immunity is set to zero (A, a = 0), to target infected red blood cells (B, a = 150, b = 100) or to remove merozoites (C, a = 7200, b = 100). Immunity targeting short-lived merozoites increases the parameter space favorable to synchronous infections.