Rarely, malaria is acquired through non-mosquito-borne transmission. Infection can follow transfusion of malaria-tainted blood products as well as exposure to RBC-contaminated tissues (such as bone marrow) and transplanted organs. Malaria parasites can also pass through the placental barrier, sometimes leading to congenital infection in newborns.
In the absence of reinfection, malaria that recurs following treatment falls into two categories: relapse and recrudescence. Malaria relapse is seen exclusively in P. vivax and P. ovale, and represents a reseeding of the bloodstream by dormant parasites (called hypnozoites) contained in the liver. Plasmodium falciparum and P. malariae do not produce hypnozoites. The recurrence of malaria in these species, conversely, reflects the proliferation of surviving blood-stage parasites from an earlier infection, an event called recrudescence.
Only female mosquitoes of the genus Anopheles transmit human malaria. The genus includes roughly 400 species of Anopheles mosquitoes worldwide, of which 60 species are malaria vectors, and some 30 species are of major importance (Bruce-Chwatt, 1985). Anopheles gambiae, the principal malaria mosquito in sub-Saharan Africa, is a particularly effective malaria vector because of its strong preference for feeding on humans and its long life compared with some other anopheline species. Up to 10 percent of A. gambiae in certain areas of Africa carry P. falciparum sporozoites at a given time. A small number of such mosquitoes present a greater hazard to humans than a large number of other anopheline species (such as forest vectors) less likely to bite humans, and in whom sporozoite rates are as low as 0.1 percent.
Most anopheline mosquitoes live in tropical and subtropical regions, although some species also thrive in temperate climates and even survive the Arctic summer. As a rule, they do not breed at altitudes above 2,000-2,500 meters. Within these geographic bounds, there are many areas free of malaria, however, because transmission is highly dependent on local environmental and epidemiologic conditions.