Four main kinds of plasmodium cause human malaria:

• Plasmodium falciparum is the commonest cause of malaria in Africa and is capable of causing complicated and fatal infections. It causes deaths, mainly in young children, and is particularly dangerous to non-immune adults. It is limited to tropical regions because the parasite requires high temperatures to mature inside mosquitoes.
• P. vivax is the commonest cause of malaria worldwide, but it is rarely deadly.
• P. ovale and P. malariae are less common conditions, causing few deaths.

Mixed infections with more than one species are common in some parts of the world.

Plasmodia have a complex life cycle, evolving through different stages as they mature and pass through mosquito and human hosts. Once injected into a human host, the parasite migrates to the liver, where it multiplies in liver cells for one to two weeks before generating a new form of the parasite that infects and grows in red blood cells. This ‘erythrocytic’ stage causes the main symptoms of malaria with synchronised reproduction cycles that lead to waves of red blood cell destruction, anaemia and fever, recurring at intervals of 48 or 72 hours. These are referred to as ‘tertian’ and ‘quartan’, respectively.

These cyclical patterns of fever do not always occur with P. falciparum because multiple broods of the parasite, each of which is tertian, can coexist in a person without being synchronised. P. vivax and P. ovale are regularly tertian, while P. malariae is quartan.