What should you know about bacillus cereus gastroenteritis?

Bacillus cereus is a bacterium that can grow in food when it is stored or handled poorly. Some strains produce toxins before the food is eaten, causing prominent vomiting; others produce toxins in the intestine and cause watery diarrhoea and cramps. Symptoms may begin abruptly after contaminated food, and fever is not always present. The illness is usually short-lived, but repeated vomiting or diarrhoea can remove fluid and salts from the body.

Foods that have been cooked and then left warm for too long can allow this bacterium to multiply. The symptoms overlap with other forms of gastroenteritis, so the food history and timing help a clinician judge the cause.

Take frequent small sips of water or an oral rehydration drink, especially after each episode of vomiting or diarrhoea. Rest, and return to light foods when nausea settles; avoid alcohol and foods that clearly worsen symptoms. Wash hands with soap and water and avoid preparing food for other people while unwell.

Do not use leftover antibiotics or medicines to stop diarrhoea without advice. A clinician may recommend testing or treatment when symptoms are severe, prolonged, or affecting someone vulnerable. Read about [Dehydration](/conditions/dehydration/) and [Gastroenteritis](/conditions/gastroenteritis/) for related guidance.

Seek urgent medical help if you cannot keep fluids down, feel faint or confused, pass very little urine, have blood in vomit or stool, or develop severe or worsening abdominal pain. A high fever, marked weakness, or signs of dehydration also need prompt assessment.

Arrange medical advice if symptoms are not easing, recur after eating, or affect an infant, an older adult, a pregnant person, or someone with a long-term illness. Diarrhoea, nausea, fever and loss of appetite can have several causes.

Which food or exposure could explain the timing? Do I need a stool test? How should I replace fluids if vomiting continues? Which medicines should I avoid? What symptoms would mean I need urgent review?