The microbial pathogens of greatest concern to human health during water-based recreational activities include viruses, bacteria and parasites. Viruses, bacteria, and parasites that live in the intestinal tract of humans and other animals pose the greatest risk to recreational water quality. Viruses are organisms that can cause infectious disease such as Hepatitis A. They grow and multiply in living cells and are too tiny to be seen with an ordinary microscope.
Many viruses attack the gastrointestinal or respiratory systems of the human body.
There are hundreds of bacteria that are of concern for human health. Parasites are organisms that live inside other organisms and depend on a host for their survival and reproduction. Parasites include several thousand types of organisms with many that result in waterborne illness such as giardia and cryptosporidium. Sometimes there are harmful viruses, bacteria and parasites in the water that can cause ear and eye infections, gastrointestinal illness, or skin disease and irritation. Viruses, bacteria, and parasites can enter waterbodies through a variety of routes including runoff from agricultural and urban areas into surface waters and through leaching into groundwater.