T his chapter discusses a number of diseases of major importance that are not dealt with elsewhere: acquired immune deficiency syndrome (AIDS), a disease characterized by a defect in the body's natural immune system; plague, of great historical importance but fortunately now uncommon in the United States; three infectious diseases—Hantavirus, tularemia and Rocky Mountain Spotted Fever; and seven tropical diseases that afflict millions of people in the warmer regions of the world and that occasionally occur elsewhere—malaria and yellow fever, leishmaniasis, trypanosomiasis, filariasis, schistosomiasis, and leprosy.

Tropical Diseases

M ost people living in the temperate climates of North America and Europe are spared the ravages of some of the most lethal and debilitating diseases known to mankind. They include malaria, which probably has killed more people than any other disease in history, yellow fever, leishmaniasis, trypanosomiasis, filariasis, schistosomiasis, and leprosy.

While many persons probably have never heard of some of these diseases and at least a few doctors might have trouble in diagnosing the symptoms, they affect hundreds of millions of people each year and could pose a threat to persons living in any part of the world. They are generally classed as tropical diseases, but so-called tropical or exotic diseases have been prevented from spreading into temperate regions partly because of alert medical care and preventive measures by public health experts. Malaria, for example, has been found as far north as the Arctic Circle, as far south as the tip of South America, and at one time was a disease of epidemic proportions in such northern cities as Philadelphia and London. These diseases have altered the course of history, ending the life of Alexander the Great as he tried to conquer the world, nipping in the bud Napoleon's plans to retake Canada from the English, defeating French efforts to build the Panama Canal, and contributing to the black-slave trade between Africa and the Americas.

The major tropical diseases are caused by a variety of organisms, including viruses, protozoa, and worms. Some are transmitted by insect bites, some by contact with contaminated water, and others, like leprosy, are spread by means that remain a mystery despite centuries of medical experience with millions of cases of the disease. Space does not permit detailed discussion of all tropical diseases; only those regarded by medical authorities as among the most significant to world health are described in this chapter.