Diarrhoea is usually a symptom of a bowel infection (gastroenteritis), which loose, watery stools and stomach pain that occur more frequently than usual.
Diarrhoea is usually caused by a virus, or sometimes, contaminated food. Less frequently, it can be a sign of another disorder, such as inflammatory bowel disease or irritable bowel syndrome.
Most cases clear on their own. Some infections may need antibiotics. Severe cases can cause enough dehydration to require intravenous fluids.