Mosquitoes harboring sporozoites within their salivary glands inoculate them into the bloodstream of a vertebrate host during feeding. The sporozoites are carried by the bloodstream to the liver, where they enter hepatocytes and begin a phase of asexual reproduction known as the exoerythrocytic cycle. Merozoites are produced and these enter the bloodstream after the hepatocyte ruptures. The merozoites penetrate erythrocytes and begin a phase of asexual multiplication called the erythrocytic cycle. Repeated rounds of erythrocytic schizogony occur, at regular intervals.
Some merozoites invade erythrocytes and develop into male and female gametocytes, which circulate in the blood until ingested by a mosquito during a blood meal. Male gametocytes divide within the gut of the mosquito to form clusters of microgametes, by a process called exflagellation. The microgametes fertilize macrogametes that were also acquired with the blood meal and form a zygote called an ookinete. The ookinete penetrates the gut wall of the mosquito and develops intracellularly into an oocyst. The oocyst produces large numbers of sporozoites and ruptures, releasing the sporozoites into the hemocoel, where they migrate to the salivary glands.
In host-parasite relationships the distinction between definitive and intermediate hosts is usually made on the basis of where sexual reproduction occurs. In Plasmodium the sexual stages are produced in the vertebrate host, but fertilization takes place in the insect host. A case can be made for terming either host the definitive host! One way to resolve this problem of terminology is to simply referring to these as the vertebrate host and the vector.