Most cases of bacterial gastroenteritis are caused by food-borne enteric microorganisms, such as Salmonella and Campylobacter; however, it is also important to understand the risk of exposure to pathogens via recreational waters.  This is especially the case in watersheds where human or animal wastes are discharged to streams and downstream waters are used for swimming or other recreational activities. Other important pathogens other than bacteria include viruses such as rotavirus, hepatitis A and hepatitis E and protozoa like giardia, cryptosporidium and Naegleria fowleri. Due to the difficulties associated with monitoring pathogens in the environment, risk assessments often rely on the use of indicator bacteria.