Birds are worldwide infected with merozoites/schizonts of various Plasmodium species. In general there are no severe symptoms in infections (e.g., with P. relictum, P. cathemerium, or Pc elongatum). Apparently birds and these protozoans have a long history of coexistence. However, in nonadapted hosts (e.g., penguins or birds imported from far away) severe symptoms and death may occur. Vectors are – depending on the Plasmodium species – mosquitoes of the genera Aedes, Anopheles, or Culex.