The genus Plasmodium is a member of the order Haemosporidia. It is the largest genus within this order and currently consists of over 250 species. They cause malaria in many different vertebrates. The species in this genus are entirely parasitic with part of their life cycle spent in a vertebrate host and another in an invertebrate host - usually a mosquito. Vertebrates infected by members of this genus include mammals, birds and reptiles. Host range among the mammalian orders is non uniform. At least 29 species infect non human primates; rodents outside the tropical parts of Africa are rarely affected; a few species are known to infect bats, porcupines and squirrels; carnivores, insectivores and marsupials are not known to act as hosts. The listing of host species among the reptiles has rarely been attempted. Ayala in 1978 listed 156 published accounts on 54 valid species and subspecies between 1909 and 1975. The regional breakdown was Africa: 30 reports on 9 species; Australia, Asia and Oceania: 12 reports on 6 species and 2 subspecies; Americas: 116 reports on 37 species. Diagnostic criteria of the order Haemosporida Currently there are ~550 species recognised in this order organised into 17 genera. The diagnostic criteria of this family are: macrogametes and microgamonts develop independently syzygy is absent microgametocyte produces 8 flagellated microgametes zygote is motile (known as an ookinete) conoid present in ookinete stage only sporozoites naked in oocyst (that is without a sporocyst) heteroxenous: merogony and gamogony occur in vertebrate host and fertilization and sporogony in definitive host (a blood sucking insect) hemozoin pigment produced in some genera (including Plasmodium) Diagnostic criteria of the genus Plasmodium Merogony occurs both in erythrocytes and other tissues Merozoites, schizonts or gametocytes can be seen within erythrocytes and may displace the host nucleus Merozoites have a "signet-ring" appearance due to a large vacuole that forces the parasite’s nucleus to one pole Schizonts are round to oval inclusions that contain the deeply staining merozoites Forms gamonts in erythrocytes Gametocytes are 'halter-shaped' similar to Haemoproteus but the pigment granules are more confined Hemozoin is present Vectors are either mosquitoes or sandflies (Lutzomyia). Vertebrate hosts include mammals, birds and reptiles Note Mammalian erythrocytes do not possess a nucleus. Although it has been suggested that the nucleus was lost in the erythrocytes better to enable them to traverse capillaries evidence for this is lacking. It appears that this loss along with