Swimming in contaminated water may result in skin, eye, ear, and certain intestinal infections, particularly if the swimmer's head is submerged. Generally for infectious disease prevention, only pools that contain chlorinated water can be considered safe places to swim. In certain areas, fatal primary amebic meningoencephalitis has occurred following swimming in warm dirty water. Swimmers should avoid beaches that might be contaminated with human sewage, or with dog feces. Wading or swimming should be avoided in freshwater streams, canals, and lakes liable to be infested with the snail hosts of schistosomiasis (bilharziasis) or contaminated with urine from animals infected with Leptospira. Biting and stinging fish and corals and jelly fish may provide a hazard to the swimmer. Never swim alone or when under the influence of alcohol or drugs, and never dive head first into an unfamiliar body of water.