How many types of malaria infect humans?
There are four main types of malaria which infect humans: Plasmodium falciparum, P. vivax, P. malariae and P. ovale. P. ovale additionally can be split into two sympatric sub-species, P. o. curtisi and P. o. wallikeri. Each of these five kinds has a subtly different life cycle which results in slight variations in symptoms and also in treatment. For more information on this, check out the Q&As on malaria symptoms and malaria treatment; CDC is also a site worth checking out.
Additionally to the four species above, there are observations of a number of other Plasmodium species being able to infect humans, although much less frequently. The most reported of these is P. knowlesi, found in SE Asia, which usually infects macaque monkeys but is capable of crossing over into humans and causing severe quotidien malaria, and may even result in death. The number of cases of P. knowlesi appears to be on the rise in some regions, although the cause of this is not quite clear. For an introduction to P. knowlesi, you should read Christina Faust’s blog.
The list of other species of Plasmodium that have been reported to infect humans (sometimes only experimentally in the lab) includes P. brasilianum, P. cynomolgi, P. eylesi, P. inui, P. rhodiani, P. schwetzi, P. semiovale, P. simium and P. tenue.