Plasmodium malariae is a parasitic protozoan that causes malaria in humans. It is one of several species of Plasmodium parasites that infect other organisms as pathogens, also including Plasmodium falciparum and Plasmodium vivax, responsible for most malarial infection. Found worldwide, it causes a so-called "benign malaria", not nearly as dangerous as that produced by P. falciparum or P. vivax. The signs include fevers that recur at approximately three-day intervals – a quartan fever or quartan malaria – longer than the two-day (tertian) intervals of the other malarial parasite.

Malaria has been recognized since the Greek and Roman civilizations over 2,000 years ago, with different patterns of fever described by the early Greeks. In 1880, Alphonse Laveran discovered that the causative agent of malaria is a parasite. Detailed work of Golgi in 1886 demonstrated that in some patients there was a relationship between the 72-hour life cycle of the parasite and the chill and fever patterns in the patient. The same observation was found for parasites with 48-hour cycles. Golgi concluded that there must be more than one species of malaria parasite responsible for these different patterns of infection.

Each year, approximately 500 million people will be infected with malaria worldwide. Of those infected, roughly two million will die from the disease. Malaria is caused by six Plasmodium species: Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale curtisi, Plasmodium ovale wallikeri, Plasmodium malariae and Plasmodium knowlesi. At any one time, an estimated 300 million people are said to be infected with at least one of these Plasmodium species and so there is a great need for the development of effective treatments for decreasing the yearly mortality and morbidity rates.

P. malariae is the least studied of the six species that infect humans, in part because of its low prevalence and milder clinical manifestations compared to the other species. It is widespread throughout sub-Saharan Africa, much of southeast Asia, Indonesia, on many of the islands of the western Pacific and in areas of the Amazon Basin of South America. In endemic regions, prevalence ranges from less than 4% to more than 20%, but there is evidence that P. malariae infections are vastly underreported.

The Centers for Disease Control (CDC) has an application that allows people to view specific parts of the world and how they are affected by Plasmodium vivax, and other types of the Plasmodium parasite. It can be found at the following link: http://cdc.gov/malaria/map/index.html.

P. malariae can infect several species of mosquito and can cause malaria in humans. P. malariae can be maintained at very low infection rates among a sparse and mobile population because unlike the other Plasmodium parasites, it can remain in a human host for an extended period of time and still remain infectious to mosquitoes.

The vector of transmission of the parasite is the female Anopheles mosquito, but many different species have been shown to transmit the parasite at least experimentally. Collins and Jeffrey report over thirty different species, which vary by geographic region.

Information about the prepatent period, or the period of time between the infection of the parasite and demonstration of that parasite within the body, of P. malariae associated malaria is limited, but the data suggests that there is great variation, often the length of time depending on the strain of P. malariae parasite. Usually, the prepatent period ranges from 16 to 59 days.