What stage of the Plasmodium life cycle is injected into humans by mosquitoes?

The stage of the Plasmodium life cycle that is injected into humans by mosquitoes is the sporozoite. When an infected female Anopheles mosquito bites a human, it injects saliva that contains these sporozoites into the bloodstream.

Sporozoites are motile forms of the parasite that travel to the liver, where they invade liver cells, mature, and eventually reproduce. This marks the initiation of the host's infection. Understanding this part of the life cycle is crucial, as the sporozoite stage is key to the transmission of malaria from the vector to the human host.

The other stages, such as trophozoites, oocysts, and merozoites, occur at different phases in the Plasmodium life cycle but are not involved in the initial transmission to humans. Trophozoites are the active feeding stage within red blood cells, oocysts are the encysted form that develops in the mosquito's gut after it has ingested blood, and merozoites are released back into the bloodstream from the liver, continuing the infection process. Thus, while they play significant roles in the malaria life cycle, only sporozoites are responsible for the initial infection process through mosquito