Human Health and Water

All living creatures, including humankind, need water for survival. Humans directly and indirectly consume water for drinking, cooking, and food production. They use it for bathing, household uses, industry and manufacturing, and waste disposal. Humans also use water environments for recreation, tourism, and ecosystem management.

Without food, a person in excellent physical condition might survive 6 weeks; without water, survival potential is measured in days. Dieticians recommend that adults drink eight 8-ounce glasses (nearly 2 liters) of water daily. To assure good health, this adequate intake of water must be of satisfactory sanitary quality.

But a multitude of different microorganisms , or microbes, can affect the quality of a water supply. This article describes the different types of microbes and their relative size, potential sources, and occurrence in water, as well as the pathways and relative sensitivity of the human population to microbial infection. It concludes with a brief overview of international public health concerns, including mosquito-borne diseases.

Microbes and their Occurrence in Water

The most common microbes found in water fall into four categories: bacteria, protozoa, viruses, and helminths. A few examples are given in the table. Some of these pathogens may be found naturally in the water supply, and others may be introduced into the water supply through human and animal waste contamination.


These microbes lack a nucleus and have a cell wall composed of a protein sugar molecule. Bacteria average 0.5 to 1 micron in diameter and 4 to 20 microns in length. (A micron is one-millionth of a meter.) Bacteria are found everywhere, covering all surfaces, and therefore can be found naturally in both groundwater and surface water . Pathogenic (disease-causing) bacteria found in water supplies generally come from human and animal waste contamination.

Human Health and Water

Disease Specific Agent Reservoir Symptoms in Brief
Diarrhea Enteropathogenic Escherichia coli O157:H7 Intestines of animals and infected persons Cramping, vomiting, diarrhea (occasionally bloody), fever, dehydration
Salmonellosis Salmonella typhimurium Animals and eggs Abdominal pain, diarrhea, chills, fever, vomiting and nausea
Typhoid Fever Salmonella typhosa or typhi Feces and urine of typhoid carrier or patient Fever, usually rose spots on the trunk, diarrheal disturbances
Paratyphoid Fever Salmonella paratyphi (ABC) Feces and urine of carrier or patient Fever, diarrheal disturbances, sometimes rose spots on trunk, other symptoms
Shigellosis (Bacillary dysentery) Shigella Feces of carrier and infected persons Acute onset diarrhea, fever, tenesmus, frequent stools containing blood and mucus
Campylobacter Enteritis Campylobacter Jejuni Chickens, swine, dogs, cats, human, raw milk, contaminated water Watery diarrhea, abdominal pain, fever chills, nausea, vomiting, blood in stool
Cholera Vibrio cholerae, Vibrio comma Feces, vomitus; carriers Diarrhea, rice-water stools, vomiting, thirst, pain, coma
Amebiasis (Amebic dysentery) Entamoeba histolytica Bowel discharges of carrier, and infected person; possibly also rats Diarrhea or constipation, or neither; loss of appetite, abdominal discomfort; blood, mucus in stool
Cryptosporidiosis Cryptosporidium Farm animals, human, fowl, cats, dogs, mice Mild flulike symptoms, diarrhea, vomiting, nausea, stomach pain
Giardiasis Giardia lamblia Bowel discharges of carrier and infected persons; dog, beaver Prolonged diarrhea, abdominal cramps, severe weight loss, fatigue, nausea, gas, fever is unusual
Viral Gastroenteritis Rotaviruses, Norwalk agent, etc Human, feces, or sewage Nausea, vomiting, diarrhea, abdominal pain, low fever
Infectious Hepatitis Hepatitis A Feces from infected persons Fever, nausea, loss of appetite; possibly vomiting, fatigue, headache, jaundice
Schistosomiasis Schistosoma Venous circulation of human; urine, feces, dogs, cats, pigs, cattle, horses, field mice, wild rats, water buffalo Dysenteric or urinary symptoms, rigors, itching on skin, dermatitis


These are single-celled organisms with a membrane-enclosed nucleus. Protozoa vary between 2 and 70 microns in size. Protozoa occur mostly in aquatic habitats, such as oceans, lakes, rivers, and ponds. However, protozoa can make their way into groundwater through a hydraulic connection between a surface-water source and an aquifer , through springs , or through improperly constructed wells.


Viruses are genetic material (RNA or DNA) surrounded by a protein protective coating. Viruses are considerably smaller than bacteria, and vary in size from 0.02 to 0.1 micron. As with bacteria, most pathogenic viruses found in water supplies, both surface water and groundwater, come from human and animal waste contamination.


This category includes intestinal worms and worm-like parasites. Helminth eggs are about 40 microns or larger in size. Helminths in

Drinking water in developing countries often is scarce and frequently is contaminated. This Liberian man in western Africa benefits from clean water supplied by a new well constructed with the aid of UNICEF (United Nations International Children's Fund).
Drinking water in developing countries often is scarce and frequently is contaminated. This Liberian man in western Africa benefits from clean water supplied by a new well constructed with the aid of UNICEF (United Nations International Children's Fund).
water supplies, typically in surface-water sources, come from human and animal waste contamination.

Sources of Microbial Contamination.

Examples of how human and animal waste may contaminate a water supply include, but are not limited to, the following:

  • Failure of an on-site sewage disposal system (e.g., septic system) that causes direct infiltration to groundwater and/or provides runoff to surface water;
  • Discharge of untreated or improperly treated sewage to rivers and reservoirs, such as during operational malfunctioning or during heavy storms with excessive stormwater runoff;
  • Overapplication of sewage plant sludge on agricultural fields (known as land application), causing contaminated water to run off to surface water and/or infiltrate to groundwater;
  • Runoff of animal wastes to surface water from pastures and rangelands; and
  • Infiltration to groundwater from high concentrations of animal waste from confined animal feedlots.

Microbial Pathways and Human Health Effects

All waterborne pathogens are transmitted to humans through drinking or otherwise ingesting contaminated water. Therefore, the keys to breaking the process of transmission of these microbes are to (1) protect the water source by preventing contamination from occurring, and (2) protect the population from ingesting contaminated water by treating a contaminated supply and limiting its use.

Three examples of recent widespread outbreaks of waterborne illness illustrate the worldwide need for adequate sanitation and safe drinking water. In 2000, a cholera outbreak in South Africa caused dozens of deaths

India's Ganges River is one of the most polluted waterways in the world. But its religious significance annually brings millions of visitors—both Indian and foreign—to its banks and bathing ghats, where their contact with the microbe-laden water poses a widespread public health risk.
India's Ganges River is one of the most polluted waterways in the world. But its religious significance annually brings millions of visitors—both Indian and foreign—to its banks and bathing ghats, where their contact with the microbe-laden water poses a widespread public health risk.
and more than 11,000 illnesses. The cholera episode was due to poor sanitation and contaminated water sources with inadequate drinking-water treatment. In 1999, a contaminated shallow well with no disinfection caused hundreds of cases of E. coli and Campylobacter jejuni at New York state's Washington County Fair. In 1993, Cryptosporidium in inadequately filtered drinking water caused more than 400,000 residents of Milwaukee, Wisconsin to fall ill.

In the United States, occasional outbreaks of gastroenteritis caused by unknown agents lead to thousands of cases of illness each year. Other common causes of waterborne disease in the United States include Giardia, Shigella, hepatitis A, Norwalk agent, Cryptosporidium, Campylobacter, Salmonella, and E. coli.

The chances that a human may develop acute, chronic, or delayed health effects from ingestion of waterborne microbes depends on the dose and virulence or toxicity of the microbes, and the natural and acquired resistance of the human to the microbes. The populations most susceptible to water-borne illness are the very young, the elderly, and those with compromised immune systems, such as AIDS patients and cancer patients undergoing radiation and chemotherapy.

In most waterborne illness outbreaks, secondary cases of illness are transmitted through the fecal–oral route. In other words, an infected person will shed the microbes through feces. He or she may use the restroom, not wash his or her hands properly, and then prepare food. The food is now contaminated, and the microbe is transmitted to the person consuming the food, thereby spreading the disease. Hence, hand-washing is not only a personal health measure but also a critical step in public health protection.

Case Study: Walkerton

E. Coli Outbreak From May through December of 2000, seven people died from an outbreak of E. coli O157:H7 in Walkerton, Ontario, Canada. The city reported 160 confirmed cases of E. coli, more than 400 unconfirmed cases, and more than 2,300 people ill with gastrointestinal illness. What could be the cause of an outbreak of this magnitude?

Heavy rainfall washed cattle manure infected with E. coli O157:H7 and Campylobacter jejuni into a shallow public-supply well in Walkerton. Although the manure had been spread on agricultural land near the well in accordance with proper agricultural practices, the contaminants had passed from the soil down into the aquifer that fed the well.

Officials found that the city's water-system operators had engaged in a series of improper monitoring practices (e.g., not collecting water samples from the appropriate sampling sites) and had falsified reports. Therefore, the contamination went undetected. An investigation published in 2002 offered recommendations for regulatory and policy reform in order to strengthen Ontario's protection of drinking water.

The Walkerton E. coli O157:H7 outbreak is a chilling reminder that communities take high-quality drinking water for granted. Key factors in preventing the occurrence of similar outbreaks involve minimizing the chance of microbial contamination in water supplies: namely, protecting water sources; replacing aging portions of the water-supply infrastructure; properly maintaining existing structures; and improving training for watersystem operators.

Using tiny crustaceans that eat mosquito larvae is one method of controlling mosquitoes that cause dengue fever. This Vietnamese woman inspects mesocyclopes in a cup of water drawn from a village water tank. Dengue is a serious public health threat for two-thirds of the world's population.
Using tiny crustaceans that eat mosquito larvae is one method of controlling mosquitoes that cause dengue fever. This Vietnamese woman inspects mesocyclopes in a cup of water drawn from a village water tank. Dengue is a serious public health threat for two-thirds of the world's population.

Mosquito-Borne Diseases

Water plays a role in mosquito-borne diseases by providing breeding grounds for mosquito larvae. One category of mosquito-borne infection known as dengue (including dengue hemorrhagic fever) has intensified and expanded in recent years, becoming a major international public health concern. Dengue is found in tropical and subtropical regions, predominantly in urban and semiurban areas. The world distribution of dengue encompasses Asia, Africa, the Pacific, and the Americas. In 1997, dengue was considered by public health authorities as the most important mosquito-borne viral disease affecting humans; its global distribution was comparable to that of malaria. As of 2002, two-fifths of the world's population (2.5 billion) was considered at risk from dengue.

The mosquitoes responsible for dengue commonly breed in human-made containers, such as earthenware jars, metal drums, and concrete cisterns used for domestic water storage, as well as used automobile tires and other items that collect rainwater. Inadequate water, sewage, and waste management systems can increase the mosquito populations. Other reasons for the dramatic global emergence of dengue as a public health problem include uncontrolled urbanization and concurrent population growth; increased airplane travel; and deteriorating public health infrastructure.

Although there is a small but insignificant risk for dengue outbreaks in the continental United States, American attention recently has focused on the spread of the West Nile virus. Until 1999, this mosquito-borne virus had not been documented in the Western Hemisphere. Previously it commonly had been found in humans, birds, and other vertebrates in Africa, Eastern Europe, West Asia, and the Middle East. As of late 2002, evidence of infection in birds, humans, mosquitoes, and other animals (primarily horses) had been documented in 43 U.S. states and the District of Columbia, with more than 3,500 human cases of the virus and more than 200 deaths.

Global Availability of Safe Water

Safe water and the prevention of waterborne disease are public health priorities in most developed countries, where clean water generally is available for about one-third of the world's population. However, water-related human health problems in developing countries are daunting. Global estimates of the population in developing countries that lack access to safe drinking water range from 1.1 to 1.4 billion. The number of people without basic sanitation services is estimated at 2.4 to 2.9 billion.

The consequences of lack of safe water are severe. The United Nations' World Health Organization estimates that more than 3 billion cases of illness and 5 million deaths—the majority children—can be attributed annually to unsafe water. The death rate for children alone is estimated at one every 8 seconds.

Those numbers may rise in the early years of this century. World population surpassed 6 billion in 1999. It is expected to top 7 billion in 2013, and 8 billion by 2028. Most of the increases are expected to be in developing countries, increasing pressure on already inadequate water resources.

Waterborne Diseases.

Cholera and typhoid claimed millions of lives from the Middle Ages (a period from the fifth century to fifteenth century) to the middle of the nineteenth century. Scientists in the nineteenth century linked cholera to disposal of sewage in water that later was used for drinking. A cholera outbreak in London, England in 1854 was traced to a single contaminated well.

Significant disease reductions occurred near the end of the nineteenth century when filtering municipal drinking water became an accepted practice. Further reductions came early in the twentieth century when chlorination of water supplies was introduced. Yet disease in developing countries remains highly pervasive, even in the twenty-first century. Water-caused intestinal infections today often are associated with impoverished regions with inadequate treatment of drinking water and sewage. For example, more than 137,000 new cases of cholera were reported in 2000, 85 percent of which were in Africa.

An estimated 1.5 billion persons suffer infections with intestinal helminths each year. Another billion suffer diarrheal diseases (cholera, E. coli, giardiasis, and others). Such diseases are linked to unsanitary excreta disposal, poor personal and domestic hygiene, and in the case of diarrheal diseases, unsafe drinking water. Persons can become disease victims by drinking contaminated water, washing with it, or bathing in it, as well as through disease vectors (e.g., insects) that breed in it.

During the International Drinking Water Supply and Sanitation Decade (from 1981 to 1990), concentrated efforts were made to extend services to unserved and underserved populations in developing countries. Some progress was made, but the overall effort failed. Underestimation of the problem's extent, inadequate funding, and the need for extensive hygiene and sanitation education among affected populations were contributing factors. The World Health Organization, the World Bank, and the entire international community must focus on capacity-building in developing countries. Only through these efforts can the ongoing cycle of death and disease be diminished.

SEE ALSO Clean Water Act ; Developing Countries, Issues in ; Drinking Water and Society ; Drinking-Water Treatment ; Fresh Water, Natural Composition of ; Fresh Water, Natural Contaminants in ; Human Health and the Ocean ; Infrastructure, Water-Supply ; Legislation, Federal Water ; Microbes in Groundwater ; Microbes in Lakes and Streams ; Microbes in the Ocean ; Pollution of Groundwater ; Pollution of Lakes and Streams ; Pollution Sources: Point and Nonpoint ; Safe Drinking Water Act ; Septic System Impacts ; Supplies, Protecting Public Drinking Water ; Supplies, Public and Domestic Water ; Survival Needs ; Utility Management ; Wastewater Treatment and Management .

Karen E. Kelley (microbes)

Edward F. Vitzthum (global perspective)


Barzilay, Joshua I., Winkler G. Weinberg, and J. William Eley. The Water We Drink. New Brunswick, NJ, and London, U.K.: Rutgers University Press, 1999.

Gleick, Peter H. The World's Water 2000–2001: The Biennial Report on Freshwater Resources–2001. Washington, D.C.: Island Press, 2000.

O'Connor, Dennis R. Report of the Walkerton Inquiry: Part I—The Events of May 2000 and Related Issues; Part 2—A Strategy for Safe Drinking Water. Toronto, Ontario: Walkerton Public Utilities Commission, 2002.

Salvato, Joseph A. Environmental Engineering and Sanitation. New York: John Wiley & Sons, 1992.

Internet Resources

EPA Ground Water and Drinking Water. U.S. Environmental Protection Agency. <> .

Infectious Disease Information. Centers for Disease Control and Prevention. <> .

MacKenzie, William R. et al. "A Massive Outbreak in Milwaukee of Cryptosporidium Infection Transmitted through the Public Water Supply." New England Journal of Medicine 331 (3):161–167 (July 1994). <> .

"Public Health Dispatch: Outbreak of Escherichia coli O157:H7 and Campylobacter among Attendees of the Washington County Fair—New York, 1999." CDC MMWR Weekly 48 (36): 803 (17 September 1999). <> .

World Health Organization. Disease Outbreaks News. World Health Organization Communicable Disease Surveillance and Response (CSR). <> .


Travel advisories may contain warnings about diseases of concern, information on immunizations required for entering certain destination countries, and recommendations on avoiding illnesses associated with food or water. An example of a water-associated illness is gastrointestinal upset, including diarrhea, caused by drinking waters containing pathogenic microbes.

However, a common source of such symptoms is not contaminated water, but rather water that is markedly different from the water that one's body is accustomed to. For example, in many arid regions, such as the southwestern United States and Mexico, water, particularly well water, is high in constituents such as magnesium and sulfate. Although not presently a significant health concern, both of these dissolved minerals can have a laxative effect on humans.

Travelers going to areas where they are concerned about the drinking water would be best served by using bottled water. Boiling the water, or using chlorine or iodine tablets, will certainly kill any pathogenic organisms. But chlorine and iodine will have no effect on dissolved minerals, and boiling will only enhance their effects.

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