Diseases transmitted by organisms living in fresh water have been a major force in shaping both the size of the human population and the development of our civilization. Certainly, far more people have been killed by diseases transmitted by freshwater vectors than those caused by all the wars in the history of the world. This is true even in recent times; during the Vietnam War, many times more casualties resulted from malaria than from combat.
Water-related human diseases currently kill more than 5 million people a year, and some 2.3 billion people suffer from diseases linked to water. Waterborne diseases can be classified into four major categories: (1) water-borne diseases that are spread through contaminated drinking water, such as cholera; (2) water-washed diseases arising from poor sanitation, such as typhoid; (3) diseases that are directly transmitted by vectors (usually biting flies) that depend on fresh water for one or more of the stages of their life cycle, such as malaria; and (4) diseases that are transmitted to humans that involve an aquatic animal that serves as an intermediate host, usually snails, such as schistosomiasis. Other articles in the Encyclopedia concentrate on the first two categories (e.g., coliforms); this article concentrates on the latter two categories.
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