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Food and Waterborne Illness

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Causes: Microbes and their Toxins

The microbes that can contaminate food and water include a wide range of bacteriaviruses, and parasites. Some are found throughout the world while others are regional. Some are very common, others more rare. They may cause symptoms in most people who are exposed to them or only in those who are most susceptible. Regional strains of bacteria may not cause symptoms in the people who are accustomed to them but may make visitors sick. Though the affected water and food may smell, look, and taste normal, they can cause illness ranging from mild and self-limited to severe.

Norovirus is the leading cause of food and waterborne illnesses, according to the most recent numbers from the Centers for Disease Control and Prevention (CDC). It is a very contagious virus that can contaminate food and water and also be spread from person to person.

Several bacteria are also among the most common causes:

Norovirus, Salmonella, Listeria, and the parasite Toxoplasma are also considered to be among the most common causes of death due to foodborne illness. Norovirus usually causes only a mild illness, but because it affects so many people, it is among the top causes of death.

Select one of the links below to read the details about these and other microbes that have been implicated in food and waterborne illnesses:

(Note: There are many other non-infectious causes of illness from consuming food or drink, such as poisoning from fish contaminated with toxins, but these are beyond the scope of this article.)


  • Salmonella—these bacteria may contaminate raw meat, poultry eggs, and other foods. Symptoms usually develop within 12 to 72 hours after infection and can last from 4 to 7 days. The infection usually resolves without treatment or with supportive care only, but in some people, such as the very young or elderly, the diarrhea may become so severe that they require hospitalization. In those that are very ill, there is a danger that the infection may spread to the blood, and there is a risk of death. These severe cases must be treated promptly with antibiotics. Salmonella serotype Enteritidis and Salmonella serotype Typhimurium are common in the U.S. Many cases are travel-related. Salmonella typhi, which is common in developing countries, causes typhoid fever.
  • Campylobacter are relatively common bacteria. Campylobacter jejuni has been implicated in some recent outbreaks. Illnesses causing watery and/or blood stools may develop 2 to 5 days after infection and last about a week; they are usually seen with unpasteurized milk or cheese, raw or undercooked poultry, or contaminated water. Complications from the infection include Guillain-Barré syndrome and reactive arthritis. The number of cases is typically increased in spring and fall. Usually, supportive care is sufficient treatment, but some severe or prolonged cases may require antibiotics.
  • Escherichia coliE. coli are common bacteria normally found in the digestive tract of humans and animals. Most strains don't cause problems, but some produce a toxin, known as the "shiga toxin," that can cause bloody diarrhea and potentially serious infections spread from sources such as undercooked meat or from person to person. These are known as shiga toxin-producing E coli or STEC. One particularly severe strain that has been implicated in several outbreaks is designated as O157:H7. The very young and the elderly are usually the most severely affected by complications of STEC infections, such a hemolytic uremic syndrome (HUS).
  • Listeria monocytogenes—found in uncooked meats, vegetables, fruits, soft cheeses, hot dogs, and unpasteurized milk. These bacteria can grow well even at refrigerator temperatures. The illness can be serious in those with weakened immune systems and can be passed from a pregnant woman to her developing baby and cause miscarriage.
  • Vibrio—found in contaminated seafood such as raw oysters. Vibrio parahaemolyticus is the most common; Vibrio vulnificus is found in warm seawater and can be deadly, especially in those with liver disease or weakened immune systems. Vibrio cholerae is responsible for cholera.
  • Yersinia—found in undercooked pork, seafood, and unpasteurized milk; it is frequently associated with 'chitlins,' a dish prepared from intestines of hogs often served during holidays, leading to an increased number of cases in winter. The infection is associated with reactive arthritis. Yersinia enterocolitica is the most common species.
  • Shigella—of several species that exist, some produce toxin and cause reactive arthritis and hemolytic uremic syndrome. Shigella dysenteriae causes dysentery, severe, bloody diarrhea and fever.


  • Norovirus (Norwalk-like virus)—the most common cause of gastroenteritis in the U.S., causing over 50% of foodborne illnesses. It is resistant to disinfectants and is very contagious as it can be spread from person to person, by contaminated food or water, or by touching contaminated surfaces. It is a common cause of illness on cruise ships, in restaurants, and other confined populations. Most people get better in about 1 to 3 days, but there is a risk of dehydration and serious illness in young children, the elderly, and people with underlying conditions. People with severe dehydration may need to be hospitalized; antibiotics are NOT used for treatment since it is a virus, not a bacterium.
  • Rotavirus—very common in children; it can cause severe diarrhea and dehydration in this population and in the immunocompromised.
  • Hepatitis A—the number of cases in the U.S. is decreasing, but the infection is prevalent in other parts of the world; infection can be acquired from sewage-contaminated water, shellfish, and/or vegetables and other uncooked foods. A vaccine is available to prevent infection.


Common parasites causing food and waterborne illness include:

  • Toxoplasma gondii—this is a single-celled parasite that can be ingested when consuming contaminated food or water, especially undercooked pork, lamb or venison; it may also be acquired by handling contaminated cat litter, by transmission from a pregnant woman to her developing baby, and rarely may be transmitted during an organ transplantation or blood transfusion. Most healthy people who become infected are not aware of it, and if symptoms are present, they are usually mild and flu-like. Their immune systems are able to protect them against more severe forms of infection. People with weakened immune systems and infants born to mothers who become infected with the parasite during or just before pregnancy are most vulnerable to serious infection.
  • Giardia duodenalis (Giardia intestinalis, Giardia lamblia)—a very common cause of waterborne illness, it also can contaminate food and may be passed from person to person; an infected person may have few and/or intermittent symptoms.
  • Entamoeba histolytica— a parasite acquired from contaminated water or food; it can be spread from person to person.
  • Cryptosporidium parvum—also called "crypto," may be found in food and water; infection can be severe and persistent in the immunocompromised.

Other potential parasitic causes:

  • Cyclospora cayetanensis—found in contaminated food or water; causes watery diarrhea
  • Microsporidia, multiple species—infection from contaminated water can cause chronic diarrhea in immunocompromised people.
  • Cystoisospora belli—found in contaminated water and food; most common in tropical areas
  • Trichinella spiralis—causes trichinosis; rare in U.S. now but may be seen with undercooked pork and wild game; cysts of the parasite can lodge in muscles
  • Taenia solium and Taenia saginata—pork and beef tapeworms from raw or undercooked meat, contaminated food or water


Other sources of food and waterborne illnesses include toxins that are produced by some bacteria. They may include:

  • Staphylococcus aureus—can produce a toxin that causes acute symptoms of nausea and vomiting within several hours of consuming the contaminated food
  • Clostridium perfringens—these bacteria may contaminate raw meat and poultry; disease is associated with food that is not cooked, heated or stored properly. The bacteria form spores that resist high temperatures, so when food that has been cooked cools down, the bacteria can begin to grow. The ingested bacteria produce a toxin that causes an acute infection, with symptoms of intestinal cramping and diarrhea (but no fever or vomiting) that typically develop within 8 to 12 hours, lasting less than 24 hours. As with other causes of food and waterborne illnesses, the infection can become serious in those who are most vulnerable.
  • Clostridium botulinum—from contaminated food, toxin can cause paralysis and can be fatal. Illness can be seen with low acid home-canned food; toxin is destroyed by high temperatures.

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