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The Aswan Dam seriously damaged a fragile ecosystem.
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#2 - The Change in Environment May Be More Conducive to the Spread of the Pathogen or Its Vector.
* New habitats, such as standing pools of water or agricultural monocultures, can be created: in the 1950s Egypt built the Aswan High Dam to supply its growing population with fresh water from the Nile. This created ideal habitat for species of flatworms which cause schistosomiasis, a disease which affects the circulatory system, the central nervous system, and other organs.
* Predators, which once kept down the numbers of disease vectors, can be lost: thanks to the elimination of their natural predators (through original deforestation and hunting) and subsequent reforestation, the white-tailed deer population of the United States has exploded. The deer carry the tick that transmits Lyme disease, which is now the most reported vector-borne disease in the country.
* The quality and/or quantity of fresh water can be altered, allowing water-borne disease to spread: in Milwaukee in 1993, an estimated 403,000 people fell ill with cryptosporidiosis, a digestive system illness, when microbes in animal waste from farms near the watershed area contaminated drinking water drawn from Lake Michigan. This was the largest recorded waterborne disease outbreak in U.S. history.
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