Which stage of Plasmodium is injected into humans by mosquitoes?

The stage of Plasmodium that is injected into humans by mosquitoes is the sporozoite. When an infected female Anopheles mosquito bites a human, it injects saliva containing the sporozoites, which are the infective form of the parasite. These sporozoites quickly travel to the liver, where they undergo further development.

Merozoites are produced later in the life cycle after the sporozoites mature within liver cells. Trophozoites are a stage of the parasite that occurs inside red blood cells after the merozoites have entered the bloodstream. Gametes are the reproductive forms that develop in the mosquito itself and are not involved in the transmission process to humans. Thus, sporozoites are critical for initiating the infection in humans following a mosquito bite.