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Plasmodium malariae protozoan inside red blood cells in the ring-form trophozoite stage, computer illustration. P. malariae is the causative agent of quartan malaria, also known as malarial malaria. It is transmitted to humans by the bite of the female Anopheles mosquito. The illustration shows the typical morphological features of P. malariae in the ring form (early trophozoite) stage. It is smaller than the ring form of P. vivax, occupying one sixth of a red blood cell, and has a heavy chromatin dot. (KEYSTONE/SCIENCE PHOTO LIBRARY/KATERYNA KON/SCIENCE PHOTO LIBRARY)