Malaria is the disease resulting from infection by one or more of four species of protozoan parasites of the genus Plasmodium. These parasites are normally transmitted from one human host to the next by the bite of an infected female mosquito of the genus Anopheles. Although malaria has receded from many temperate regions in this century, the disease continues to be a major cause of morbidity and mortality in many tropical and subtropical countries. Three of the species — Plasmodium vivax, Plasmodium falciparum, and Plasmodium malariae - are widely distributed; the fourth, Plasmodium ovale, is principally a parasite of tropical Africa. P. vivax (the agent of benign tertian malaria) and P. falciparum (causing malignant tertian malaria) are responsible for the great majority of cases and deaths attributed to malaria throughout the world.

Malaria is characteristically paroxysmal, and often periodic. The classical clinical episode begins with chills, extends through a bout of fever, and ends with sweating, subsiding fever, a sense of relief, and, often, sleep. Between the early paroxysms the infected person may feel quite well; as the disease progresses, however, the patient may be increasingly burdened by symptoms, even in the periods between febrile paroxysms. Although infection by any species may have serious, even fatal, consequences, P. falciparum infection is particularly dangerous because of complications associated with this parasite.

The term malaria, from the Italian mala and aria ("bad air"), was certainly in use in Italy by the seventeenth century to refer to the cause of intermittent fevers thought to result from exposure to marsh air or miasma. Horace Walpole wrote home from Italy in 1740 about "[a] horrid thing called mal'aria, that comes to Rome every summer and kills one." This is said to be the first use of the term in English, and it may well have been the first appearance of the word in print (Russell et al. 1963). However, Walpole and other writers of the eighteenth century, and later, used the term to refer to the presumed cause rather than the disease. It was only after the pathogenic agents were identified at the end of the nineteenth century that usage shifted so that the term "malaria" came to refer to the disease rather than the agent.

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