Plasmodium vivax protozoan inside red blood cells, computer illustration. P. vivax is the causative agent of benign tertian malaria transmitted to humans by the bite of the female Anopheles mosquito. The illustration shows the typical morphological features of the ring-shaped trophozoite of P. vivax, used in laboratory diagnosis. The ring form (early trophozoite) measures one third of a red blood cell's diameter, it has a single chromatin dot (purple) and surrounds a vacuole (colourless).