 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.In the 1950s, a series of epidemiological studies were done in the US to determine the relationship between water quality of natural waters and the health of bathers.