Vast numbers of malaria parasites exist in a population: perhaps 10(10) in just one vertebrate host. Yet gametocytes, the only stage capable of transmission, usually constitute just a few percent or even less of the circulating parasites. Why? Parasite fitness should be intimately linked with transmission probability and infectiousness rises with gametocyte density. Here, Louise Taylor and Andrew Read propose several testable hypotheses that might explain why natural selection has not favoured variants producing more transmission stages.