The Collaborative International Dictionary of English v.0.48:

Malaria parasite \Malaria parasite\
   Any of several minute protozoans of the genus Plasmodium
   (syn. Haematozoon) which in their adult condition live in
   the tissues of mosquitoes of the genus Anopheles (which
   see) and when transferred to the blood of man, by the bite of
   the mosquito, produce malaria.

   Note: The young parasites, or sporozoites, enter the red
         blood corpuscles, growing at their expense, undergoing
         sporulation, and finally destroying the corpuscles,
         thus liberating in the blood plasma an immense number
         of small spores called merozoites. An indefinite but
         not ultimated number of such generations may follow,
         but if meanwhile the host is bitten by a mosquito, the
         parasites develop into gametes in the stomach of the
         insect. These conjugate, the zygote thus produced
         divides, forming spores, and eventually sporozoites,
         which, penetrating to the salivary glands of the
         mosquito, may be introduced into a new host. The
         attacks of the disease coincide with the dissolution of
         the corpuscles and liberation of the spores and
         products of growth of the parasites into the blood
         plasma. Several species of the parasite are
         distinguished, as Plasmodium vivax, producing tertian
         malaria; Plasmodium malariae, quartan malaria; and
         Plasmodium (subgenus Laverania) falciferum, the
         malarial fever of summer and autumn common in the
         tropics.
         [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Plasmodium vivax
    n 1: parasitic protozoan of the genus Plasmodium that causes
         malaria in humans [syn: plasmodium, Plasmodium vivax,
         malaria parasite]