The relationships between dispersal and local transmission rate of parasites are essential to understanding host–parasite coevolution and the emergence and spread of novel disease threats. Here we show that year-round transmission, as opposed to summer transmission, has repeatedly evolved in malarial parasites (genera Plasmodium and Haemoproteus) of a migratory bird. Year-round transmission allows parasites to spread in sympatric host's wintering areas, and hence to colonize distantly located host's breeding areas connected by host-migration movements. Widespread parasites had higher local prevalence, revealing increased transmission, than geographically restricted parasites. Our results show a positive relationship between dispersal and local transmission of malarial parasites that is apparently mediated by frequent evolutionary changes in parasite transmission dynamics, which has important implications for the ecology and evolution of infectious diseases.