Frequent Questions

02. What pathogens, like Giardia lamblia, does EPA regulate in drinking water, and what are their health effects?


Definition and Health Effects1


Cryptosporidium is a single-celled protozoan parasite commonly found in lakes and rivers, especially when the water is contaminated with sewage and animal waste. Cryptosporidium can cause gastrointestinal illness (e.g., diarrhea, vomiting, cramps).

Giardia lamblia

Giardia lamblia is a single-celled protozoan parasite that lives in the intestine of infected humans or animals. It is found on surfaces or in soil, food, or water that has been contaminated with the feces from infected humans or animals. 2 Giardia lamblia can cause symptoms such as nausea, cramps, diarrhea, and associated headaches.


Legionella bacteria are found naturally in the environment, usually in water. The bacteria grow best in warm water, like the kind found in hot tubs, cooling towers, hot water tanks, large plumbing systems, or parts of the air-conditioning systems of large buildings.3 Legionella bacteria in water are a health risk if the bacteria are aerosolized (e.g., in an air conditioning system or a shower) and then inhaled. Inhalation can result in a type of pneumonia known as Legionnaires disease.

Viruses (enteric)

Enteroviruses are small viruses that live in the intestines of infected humans or animals. This group includes the polioviruses, coxsackieviruses, echoviruses, and other enteroviruses. In addition to the three different polioviruses, there are 62 non-polio enteroviruses that can cause disease in humans: 23 Coxsackie A viruses, 6 Coxsackie B viruses, 28 echoviruses, and 5 other enteroviruses.4 Illness from viruses ranges from gastroenteritis caused by viruses such as rotavirus and norovirus (Norwalk-like virus) to meningitis caused by echovirus to myocarditis caused by Coxsackie B.

  1.   This health effects language is not intended to catalog all possible health effects for pathogens. Rather, it is intended to inform consumers of some of the possible health effects associated with pathogens in drinking water when the rule was finalized.
  2.   Center for Disease Control and Prevention (CDC), "Giardiasis" fact sheet
  3.   CDC, Patient Facts: Learn More about Legionnaires' disease
  4.   CDC, Non-Polio Enterovirus Infections
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