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Common causes

There are many infectious and non-infectious causes of acute and chronic diarrhoea. Viral, bacterial and parasitic infections can cause diarrhoea that lasts several days to a few weeks, although some cases may linger – causing chronic diarrhoea in those with suppressed immune systems (such as those who have HIV/AIDS, cancer, or organ transplants).

These sorts of diarrhoea are infectious, with the virus, bacteria or parasite being 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 (so called food poisoning).

Those who visit developing countries are often at risk of exposure to viral, bacterial and parasitic infections (see also Traveller's Diseases). Something as simple as contaminated ice cubes, a fresh fruit salad or food from a market can cause illness. Once a person is infected, they may then 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 day care centres, 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.

Common causes of acute diarrhoea

Parasites: The most common parasites in Australia are Giardia lamblia (Giardia), Entamoeba histolytica (E. histolytica) and Cryptosporidium parvum. These single cell parasites are found in mountain streams and lakes throughout the world and may infect swimming pools and occasionally public 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 countries, a much wider range of parasites is 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 but some of the parasites can also penetrate the skin.

Viruses: Rotavirus is the most common cause of severe diarrhoea among children. Other viruses include: noroviruses, adenoviruses, calciviruses, cytomegalovirus (CMV) and HIV. In the last couple of years norovirus has frequently been mentioned in the news for causing outbreaks of gastroenteritis on cruise ships. They also cause illness in nursing homes, schools, military establishments and anywhere that people congregate.


  • Salmonella, often found in raw eggs and poultry and may be carried by reptiles kept as pets.
  • Shigella, from faecally contaminated food and water
  • Campylobacter, from raw or undercooked poultry
  • Escherichia coli 0157:H7 (E. coli), associated with raw or undercooked hamburger/ beef. It causes bloody diarrhoea and may lead to haemolytic uraemic syndrome (red blood cell destruction and kidney failure)
  • Clostridium difficile, related to previous antibiotic treatment
  • Others: Staphylococcus aureus and species of Yersinia and Vibrio.

Acute diarrhoea may also be due to treatment with broad-spectrum antibiotics, or to other medications that cause diarrhoea 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 (pathogen). When the growth of the normal flora is inhibited, it allows easier access for a pathogen to grow and multiply. A toxin produced by the bacterium Clostridium difficile is often the culprit in antibiotic related diarrhoea.

Chronic diarrhoea (diarrhoea that lasts for more than a few weeks), sporadic diarrhoea and diarrhoea that alternates with constipation are most frequently associated with non-infectious causes. These may include:

  • Inflammatory bowel conditions such as Crohn’s disease
  • Bowel disease such as may be seen in irritable bowel syndrome
  • Malabsorption diseases such as cystic fibrosis
  • Stomach or gallbladder surgery (the rate at which the food travels through the digestive tract may change)
  • Food intolerance such as lactose intolerance or coeliac disease
  • Chemotherapy or abdominal or gastrointestinal radiation
  • Endocrine diseases such as diabetes and thyroid disease
  • Self-induced with laxatives
  • Psychological causes such as stress

Last Review Date: January 2, 2018