What Causes Stomach Flu?

Stomach flu is usually gastroenteritis caused by a virus, although bacteria, parasites, contaminated food or water, and some medicines can cause similar illness. Despite its nickname, it is not influenza, which affects the airways. The illness can spread when germs pass from an infected person, contaminated hands, surfaces, food, or drinking water into the mouth.

Viruses are a common cause, including rotavirus, particularly in young children. Bacterial gastroenteritis may follow contaminated food or water and can sometimes cause more marked fever or blood in the stool. Parasites are another possibility, especially after exposure to unsafe water or during travel. Vomiting and diarrhoea can remove fluid and salts from the body, leading to dehydration. Nausea, abdominal pain, fever, and reduced appetite may occur alongside the bowel symptoms. Symptoms alone do not reliably identify the germ, so testing is considered when the illness is severe, prolonged, or unusual.

Take frequent small sips of water or an oral rehydration solution, especially after vomiting or loose stools. Continue breastfeeding or usual milk feeds for an infant unless a clinician advises otherwise. When appetite returns, choose simple foods that you can tolerate and avoid alcohol. Wash hands with soap and water after using the toilet and before preparing food; clean frequently touched surfaces and avoid preparing meals for others while unwell. Antibiotics do not treat viral gastroenteritis and should only be used when prescribed for a bacterial cause.

Seek urgent medical advice for signs of dehydration such as very little urine, marked dizziness, unusual sleepiness, confusion, a dry mouth, or inability to keep fluids down. Prompt assessment is also needed for blood or black stool, severe or worsening abdominal pain, repeated vomiting, a high or persistent fever, or diarrhoea that is not settling. Babies, older adults, pregnant people, and anyone with a weakened immune system or a significant long-term condition may need advice sooner.

Ask whether your symptoms suggest an infection that needs testing, how much fluid you should aim to take, and which medicines are suitable for your age and health conditions. Tell the clinician when the symptoms began, whether you have travelled, eaten food that seemed unsafe, taken antibiotics recently, or had contact with someone who was ill.