3. Dams to provide hydroelectric power and store water for irrigation have had immensely beneficial consequences for many millions of people in Egypt, Ghana, Brazil and other countries. But the dams bring surface water to previously arid tropic zones; so malaria-carrying mosquitoes and water snails, the intermediate hosts of schistosomiasis, flourish. Two of the world's worst tropical diseases invade new territory. The ecological and human cost of large hydro-electric dams can be considerable. Sometimes previously sustainable agricultural ecosystems are disrupted and large numbers of people are forced to move because their habitat is submerged. At other times a hunter-gatherer culture is damaged or destroyed, as in the Amazon rain forest and in Northern Quebec. There is always a price to pay, and this must be traded off against the benefits.