Malaria is the common name for diseases caused by infection with single-celled parasites of the genus Plasmodium. They belong to the Apicomplexa. Organisms of this group are characterised by what is called an apical complex, a structural feature visible only under an electron microscope. The apical complex consists of a polar ring or rings, rhoptries (saccular organelles), micronemes (thread-shaped organelles), a conoid (cone-like structure) and subpellicular microtubules. Among the parasites of the genus Plasmodium four species have been identified which can cause disease in humans :

• Plasmodium falciparum
• Plasmodium vivax
• Plasmodium ovale
• Plasmodium malariae

Certain plasmodia (P. cynomolgi bastianelli, P. cynomolgi cynomolgi, P. brasilianum, P. schwetzi, P. inui, P. simium, P. knowlesi) which infect monkeys in the wild, may also infect humans, but because of the mild symptomatology and their rare occurrence they pose no problem in clinical practice. It is also possible that due to their rarity these zoonotic infections are missed in the normal clinical setting. Plasmodium ovale was discovered in 1922.

Sometimes subgenera are used: Plasmodium (Plasmodium) vivax, P. (P.) ovale, P. (P.) malariae and P. (Laverania) falciparum. Only P. (L.) falciparum and (L.) reichenowi belong to the subgenus Laverania. Lemurs and lower mammals are sometimes infected with Plasmodium sp. which are classified under the subgenus Vinckeia. We shall not discuss subgenera any further.

P. vivax has some subtypes: P. vivax hibernans, a strain adjusted to moderate and cold climates, P. vivax chesson and P. vivax multinucleatum. There may be other subtypes and P. vivax-like parasites. The names of the different strains of P. falciparum are not generally accepted (P. f. immaculata, quitidiamum, tenue, perniciosa, aethiopicum).

Other Plasmodium species cause infections in certain animals. Several mammals, birds and reptiles have their own form of malaria. Thus for example P. berghei and P. chabaudi are known in rodents. P. gallinaceum, P. relictum, P. elongatum and P. lophurae cause infections in birds. Chickens and even penguins may be infected, with serious consequences for these animals. Even lizards have their own form of malaria. Infections with malaria parasites in monkeys usually show few if any symptoms, with the exception of P. knowlesi infection in Rhesus monkeys. The natural host of P. knowlesi is Macaca fascicularis (the "Kra monkey"). This parasite was used early in the 20^th century in the treatment of neurosyphilis, but due to repeated passages the virulence increased to such an extent that its use was abandoned. Monkeys from Africa and Madagascar can be naturally infected with Plasmodium gonderi and P. girardi respectively. Asiatic monkeys are infected by Plasmodium inui, P. fragile, P. fieldi, P. coatneyi, P. cynomolgi, P. simiovale and P. knowlesi. P. fragile is a parasite which has Macaca sinica monkeys ("Toque monkeys") as its natural host. The natural host of P. coatneyi is the Javanese crab-eating monkey (Macaca irus). Sometimes subtypes have been described, e.g. P. inui inui and P. inui shortii; P. cynomolgi cynomolgi and P. cynomolgi bastianelli; P. knowlesi knowlesi and P. knowlesi edesoni. Monkeys from the New World may be infected by P. simium and P. brasilianum among others. Anthropoids may also be infected: P. pitheci (orang utans) and P. schwetzi as well as P. reichenowi in the case of gorillas and chimpanzees. Gibbons may be infected by P. hylobati. P. simium and P. schwetzi are very similar to P. vivax. P. simium can even be regarded as a strain of P. vivax. P. simiovale is similar to P. ovale. P. brasilianum is very similar to P. malariae and may be the same. It is possible that the P. malariae parasite was introduced at the time of the first contact of the West with South America, or was accidentally imported later at the time of the slave trade. The parasite would then have infected South American monkeys and subsequently developed features of its own as P. brasilianum.

Some of these animal malaria parasites are used in experimental research of the malaria problem. P. cynomolgi is often used as a model for P. vivax infections. Another much-used model is P. falciparum infection in Aotus monkeys. P. fragile and P. coatneyi can cause cerebral lesions in Rhesus monkeys (Macaca mulatta). These parasites are often used as an animal model for severe P. falciparum infections.