There are many infectious and non-infectious causes of acute and chronic 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 HIV/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.
Once someone is infected, the person may pass it on to others around them unless careful sanitation practices (especially thorough handwashing) 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 developing nations, may be exposed to a variety of pathogenic 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.
Common causes of acute diarrhea:
Parasites: The most common parasites in the United States are Giardia lamblia, Entamoeba histolytica, and 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' ova, but some of the parasites can also penetrate the skin.
Viruses: Rotavirus is the most common cause of severe diarrhea among children. Other viruses include Norwalk, noroviruses (also called Norwalk-like viruses), adenoviruses, calciviruses, cytomegalovirus (CMV), and HIV. The CDC estimates that Norwalk and Norwalk-like viruses cause over 20 million cases of acute gastroenteritis a year in the U.S. - because they are easily transmitted in contaminated drinking wter and on the hands of infected persons. Noroviruses have been mentioned in the news as the cause of outbreaks of gastroenteritis on cruise ships, and they may cause illness in nursing homes, schools, the military, and anywhere that people congregate.
Infection occurs when live organisms are ingested and become established in the intestinal tract, producing symptoms. Bacteria responsible for this type of illness include:
- Salmonella, often found in raw eggs, raw poultry and in pet reptiles
- Campylobacter, from raw or undercooked poultry
- Yersinia species
- Vibrio species
Intoxication occurs when the bacteria contaminating the food produces a toxin (poison) prior to consumption. Symptoms occur when the toxin is ingested; live organisms are not necessary to produce illness. This situation can occur with the following bacteria:
- Staphylococcus aureus
- Bacillus cereus
A toxin-mediated infection occurs when live organisms are consumed in food. The bacteria begin to secrete toxins once established in the intestinal tract, leading to the onset of diarrhea. Examples of this type of bacteria include:
- Shigella, from fecally-contaminated food and water
- Escherichia coli 0157:H7 (E. coli), associated with raw or undercooked hamburger/beef, unpasteurized apple cider, and spinach. It causes bloody diarrhea and may lead to hemolytic uremic syndrome, a condition associated with red blood cell destruction and kidney failure.
Acute diarrhea may also be due to treatment with broad-spectrum antibiotics or to other medications that cause diarrhea as a side effect. Antibiotic treatment can decrease the normal flora – the "good" bacteria that inhabit the gastrointestinal tract, help digest our 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 to grow and multiply. Toxins produced by the bacterium Clostridium difficile are often the culprit in antibiotic-related diarrhea.
Common causes of chronic 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:
- Inflammatory bowel conditions, such as Crohn disease
- Bowel dysfunction – such as may be seen in irritable bowel syndrome
- Malabsorption disorders – such as cystic fibrosis
- Food intolerances or sensitivities, such as lactose intolerance or celiac disease
- Stomach or gallbladder surgery (the rate at which the food travels through the digestive tract may change)
- Chemotherapy or abdominal or gastrointestinal radiation
- Endocrine diseases, such as diabetes and thyroid disease
- Colon cancer or polyps
- Carcinoid syndrome – group of symptoms seen in those with carcinoid tumors, such as may be found in the small intestine, colon, or appendix
- Self-induced with laxatives
- Anxiety or stress