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The genus Plasmodium probably evolved hundreds of millions of years ago and long before the arrival of the vertebrates (Escalante and Ayala 1995). There are over 200 species, most of which are parasites of birds. In addition, many species infect reptiles, rodents, and primates. Blood‐feeding insects, especially mosquitoes, act as their vectors. Four principal species infect humans: Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, and Plasmodium malariae. A fifth species, Plasmodium knowlesi normally parasitizes monkeys but sometimes causes fatal human infections. In addition, there are rare case reports of people becoming infected by species of Plasmodium that normally parasitize other animals. In most vertebrate hosts, Plasmodium parasites are not particularly pathogenic but in humans, they cause the disease malaria. Historical literature usually refers to it as the ague or marsh fever. The word ague derives from the Latin for sharp (acutus) because it manifests as a sharp fever (febris acuta) whilst living in a marshy district has long been associated with ill health – although this was attributed to the bad air (hence mal aria) rather than the anopheline mosquitoes flying in it.

Plasmodium falciparum, P. vivax, P. ovale and P. malariae exhibit marked differences in their biology, and molecular evidence suggests that they evolved from separate lineages. That is, they are more closely related to Plasmodium species that parasitise other animals than they are to one another. Interestingly, molecular evidence indicates that P. falciparum originated in gorillas and humans recently acquired the disease in a single cross‐species transmission. Therefore, contrary to previous assumptions, P. falciparum does not derive from chimpanzees nor did it originate in primitive human ancestors (Liu et al. 2010). It is also now apparent that P. vivax originated in Africa millions of years ago among chimpanzees, gorillas, and ancestral humans (Loy et al. 2017).

Malaria causes a higher human mortality than any other parasitic disease although the situation is improving in many countries. Between 2000 and 2017, there were significant reductions in both the number of cases and the mortality associated with malaria https://www.afro.who.int/health‐topics/malaria. In 2000, there were 233 million cases of malaria resulting in 985,000 deaths, but by 2017, the figures had declined to 219 million cases and 435,000 deaths. Although malaria transmission occurs in 91 countries and approximately 50% of the world’s population is at risk of contracting the disease, most cases occur in just 15 countries with Nigeria accounting for 27% of them and the Democratic Republic of Congo for 10%. Indeed, approximately 90% of the fatal cases occur in sub‐Saharan Africa and involve children less than 5‐years‐old. Even within a country, there can be big differences in the risks of contracting malaria. In Nigeria (population ~201 million), 76% of people live in malaria prone areas, whilst the remaining 24% live in areas of low transmission. Similarly, in Kenya (population ~47.6 million), there is virtually no risk of the disease in some regions, whilst in others the risk is high, and it is a major cause of childhood mortality. Unfortunately, at the time of writing (2021), efforts to make further reductions appear to have stalled. Regardless of whether it causes fatal disease, wherever malaria is common amongst a community it has serious socioeconomic consequences and contributes to poverty. This is because illness prevents people from working and/or they are less productive. In addition, because the disease is particularly severe in young children, they are unable to attend school and gain the education that would improve their chances in life. Some estimates suggest that the direct costs of malaria are US$12 billion per year and the indirect costs resulting from reduced economic growth substantially more. However, the causes of poverty are complex and whilst malaria undoubtedly causes immense hardship, it is not the main reason that so many people in the developing countries remain poor (Utzinger and Tanner 2013).