There is essentially no evidence that malaria existed in the United States before the advent of European explorers and settlers. It became a serious clinical and public health problem with the importation of African slaves for rice cultivation in the Carolinas and sugar cane development in the Gulf Coast plantations. Its establishement was made possible by bringing together readily available sources of infection for mosquitoes and by susceptible Anopheles mosquitoes which increased in numbers with the breaking of virgin sod and felling of forests. From the primary foci it spread along the Atlantic Coast from Florida to New England, was carried along several routes over the Appalachians and became well established throughout practically the entire Mississippi drainage. It was likewise carried by explorers and immigrants to the Pacific Coast, where it developed substantial roots.
By 1850–1860 most of the settled part of the country was highly malarious, although the hotbeds of the infection were in the South. Federal forces in the South suffered heavy casualties during the military operations of the War Between the States, while the Days of Reconstruction visited upon the South a prolonged period of intense malariousness. Although there was some improvement during the last two decades of the nineteenth century, even the discovery of the etiological agents of malaria and the elucidation of their life cycles, with the essential role played by Anopheles mosquitoes, did not greatly stimulate accurate diagnosis or the undertaking of control measures. Yet little by little as prosperity developed in the United States, as land came under more intensive cultivation and the price of quinine was greatly reduced, the heavily endemic territory began to shrink into the area from the coast of Virginia to Central Florida and westward to eastern Oklahoma and Texas.
Between 1915 and 1933 there was little change in the malaria picture except for the five-to-seven year cycles of ebb and flow. The serious economic depression beginning in 1931 greatly increased the amount and distribution of the disease, but this stimulated extensive coordinated control measures which have been responsible, at least to a considerable degree, for the marked reduction in malaria mortality and morbidity. Yet today there is extensive malaria in the South, although much less intensive than a decade ago.
In contrast to the situation which existed during the Civil War and the Spanish-American War, during World War I Army encampments in the South provided careful diagnosis and treatment for their malaria patients and established the first reliable actuarial basis between deaths and cases in the malarious Southern States. With much more experience, during World War II the malaria case rate in soldiers who had not been outside of the Continental United States has constituted a small fraction of that in the World War I in the same endemic foci. This has been due to military efforts within the posts and those of public health agencies immediately around the camps. Moreover, this joint control has also had its favorable effects on the nearby civilian communities.
Because of suppressive chemotherapy in hyperendemic malarious areas of military operations outside the Continental United States, materially aided by malaria survey and control operations, malaria is no longer a serious clinical problem within controlled military zones. Moreover, falciparum malaria is usually liquidated by adequate suppressive treatment, but vivax malaria is characteristically subject to repeated relapses up to two years. Thus, vivax cases among military personnel returning to the United States constitute a potential hazard for the establishment of exotic strains throughout the country wherever susceptible Anopheles mosquitoes are allowed to breed. In addition, civilians and merchant seamen, who have not been under strict military discipline and have been exposed to malaria overseas, constitute a grave potential risk since they frequently harbor falciparum, vivax and even at times quartan malaria parasites.