Acute diarrhoea is usually caused by a viral or bacterial infection and affects almost everyone from time to time.  A common cause in both children and adults is gastroenteritis, an infection of the bowel.

Bouts of diarrhoea in adults may also be brought on by anxiety or drinking too much coffee or alcohol. Diarrhoea may also be a side effect of a medication

NHS
Symptoms, causes, treatment and information

Macmillan Cancer Support
Diarrhoea as a result of cancer treatments

These links all come from trusted resources but if you are unsure about these or any other medical matters please contact your doctor or pharmacist for advice.