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Common Causes

There are many infectious and non-infectious causes of acute and chronic diarrhea.

Common infectious causes of diarrhea:

Viral, bacterial, and parasitic infections are associated with diarrhea that lasts several days to a few weeks, although some cases may linger – causing chronic diarrhea in those with suppressed immune systems (such as those who have AIDS, cancer, or organ transplants). These sources of diarrhea are infectious, with the virus, bacteria, or parasite shed into the stool and passed from person to person through oral contact with a contaminated surface. Eating food or drinking water that has been contaminated is the most frequent route of infection (food or waterborne illnesses).

Once someone is infected, the person may pass it on to others around them unless careful sanitation practices (especially thorough hand washing) are followed. This is especially a challenge in households with infected infants, in daycare centers, and in nursing homes. Sometimes an outbreak of bacterial or parasitic infection can be traced back to a particular restaurant or a single food item at a picnic. Sometimes it may be due to a contaminated water source.

Those who travel outside of the U.S., especially to emerging nations, may be exposed to a variety of disease-causing viruses, bacteria, and parasites. Something as simple as contaminated ice cubes, a fresh fruit salad, or food from a vendor's stall can cause illness.


  • Norovirus, also called Norwalk-like virus is the most common cause of sudden and/or severe illness of the digestive tract (acute gastroenteritis) in the U.S. It is also the leading cause of food and waterborne illnesses, according to 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. Noroviruses may cause outbreaks of gastroenteritis on cruise ships, in nursing homes, schools, the military, and anywhere that people congregate.
  • Rotavirus is the most common cause of severe diarrhea among children. The virus is spread through contact with the stool of an infected person – typically an infant. To prevent contamination, always wash hands after diaper changes and be sure to regularly clean the surfaces of all objects such as the changing table or toys that could be contaminated. Infants can also be given an oral vaccine that is up to 98% effective in preventing rotavirus infections.
  • Other examples of viruses that can cause diarrhea include adenoviruses, hepatitis A, and cytomegalovirus (CMV).

Bacteria: Bacteria can cause diarrhea by infection or by producing toxins.

Infection occurs when live bacteria are ingested and begin to grow and multiply in the intestinal tract, producing symptoms. Some bacteria commonly responsible for this type of illness include:

  • Salmonella—often found in raw eggs, raw poultry, and in pet reptiles. 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 and prompt treatment with antibiotics. Salmonella serotype Enteritidis and Salmonella serotype Typhimurium are common in the U.S. Many cases are travel-related.
  • Campylobacter—from raw or undercooked poultry, unpasteurized milk or cheese, or contaminated water. Illnesses causing watery and/or bloody stools may develop 2 to 5 days after infection and last about a week. Usually, supportive care is sufficient, but some severe or prolonged cases may require treatment with antibiotics.
  • Yersinia species—found in undercooked pork, seafood, and unpasteurized milk. Infection 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. Yersinia enterocolitica is the most common species.
  • Vibrio species—found in contaminated seafood such as raw oysters. Vibrio parahaemolyticus is the most common. Vibrio cholerae is responsible for cholera.

Some bacteria produce toxins that can cause diarrhea. Examples include:

  • Staphylococcus aureus—common bacteria found on the skin and hair as well as in the noses and throats of many people. It can cause food poisoning when a person contaminates food as it is prepared and then the food is not properly refrigerated or cooked after that. The bacteria produce a toxin that causes sudden and severe symptoms of nausea, vomiting, and diarrhea within several hours of consuming contaminated food.
  • Bacillus cereus—from a variety of foods, especially rice and leftovers that have sat out too long at room temperature; watery diarrhea and cramps can begin 6-15 hours after ingesting the toxin.
  • Clostridium difficile (C diff)—toxins produced by the bacterium Clostridium difficile are often the culprit in antibiotic-related diarrhea. Antibiotic treatment can decrease the normal flora – the "good" bacteria that inhabit the gastrointestinal tract, help digest food, and provide a protective barrier against the "bad" bacteria. When the growth of the normal flora is inhibited, it allows easier access for a pathogen like C. difficile to grow and multiply.
  • Clostridium perfringens—these bacteria may contaminate raw meat and poultry and a person may become infected after eating 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.
  • Clostridium botulinum—these bacteria cause botulism, a rare but serious disease that is often associated with food that has been improperly handled during the canning process. The bacteria produce a toxin that, in addition to vomiting and diarrhea, can cause muscle weakness, drooping eyelids, blurred vision, double vision, slurred speech, dry mouth, and difficulty swallowing. If left untreated, the disease can progress to cause paralysis of the muscles used for breathing as well as muscles of the trunk, arms, and legs. The toxin is destroyed by high temperatures. One of the most important ways to prevent foodborne botulism is to follow proper home-canning techniques.
  • Shigella—from food and water contaminated with stool; of several species that exist, some produce toxin and cause reactive arthritis and hemolytic uremic syndrome (HUS), a condition associated with red blood cell destruction and kidney failure. Shigella dysenteriae causes dysentery, severe, bloody diarrhea and fever.
  • Escherichia coli 0157:H7 (E. coli)—E. 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, especially hamburger, or from person to person. These are known as shiga toxin-producing E coli or STEC. The very young and the elderly are usually the most severely affected by complications of STEC infections, such as HUS.

Parasites: The most common parasites in the United States that cause diarrhea are:

  • Giardia species
  • Entamoeba histolytica
  • Cryptosporidium parvum

These single cell parasites are found in mountain streams and lakes throughout the world and may infect swimming pools, hot tubs, and occasionally community water supplies. Other more worm-like parasites, such as roundworms or tapeworms, may also occasionally cause infections.

In other parts of the world, especially in developing nations and warm climates, pathogenic bacteria and a much wider range of parasites are frequently encountered. These parasites include flat worms, roundworms, hookworms, and flukes. Visitors usually become infected by eating or drinking something that has been contaminated with the parasites' eggs (ova), but some of the parasites can also penetrate the skin.

Examples of non-infectious causes of diarrhea:
Chronic diarrhea, diarrhea that lasts for more than a few weeks, sporadic diarrhea, and diarrhea that alternates with constipation are most frequently associated with non-infectious causes of diarrhea. These may include diarrhea due to:

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