Overview
Stomach flu (viral gastroenteritis) is inflammation of the stomach and intestines caused by a virus. Despite the name, it is not related to influenza. It causes sudden onset of nausea, vomiting, diarrhoea, and stomach cramps. Norovirus is the most common cause. Most episodes resolve within 1 to 3 days with supportive care.
How common is it?
Norovirus alone affects approximately 3 million people in the UK each year, causing 3 million days of sick leave. It is highly contagious and spreads rapidly in closed communities (hospitals, cruise ships, care homes, schools).
Causes and risk factors
Viruses infect intestinal epithelial cells, disrupting absorption and causing increased secretion of fluid into the gut lumen.
Common risk factors
- Norovirus: contact with infected person's vomit or faeces, contaminated food or surfaces
- Rotavirus: the leading cause in children before vaccination
- Adenovirus and astrovirus: less common
- Poor hand hygiene
- Consuming contaminated food (especially shellfish, salad, or ready-to-eat food)
Symptoms
- Sudden onset of nausea and vomiting
- Watery diarrhoea (not bloody in most viral gastroenteritis)
- Abdominal cramps and pain
- Low-grade fever
- Muscle aches
- Symptoms usually begin 12 to 48 hours after exposure and resolve within 1 to 3 days
When to see a doctor
Seek medical attention for bloody diarrhoea, diarrhoea lasting more than 3 days in adults, signs of dehydration (dark urine, dizziness, dry mouth), high fever, or inability to keep any fluids down, especially in the elderly, young children, or immunocompromised.
Diagnosis
Largely clinical. Stool PCR panel can identify norovirus or rotavirus. Stool culture if bacterial infection suspected (bloody diarrhoea, persistent fever). Blood tests to assess electrolytes and dehydration in hospitalised patients.
Treatments
Oral rehydration
The primary treatment. Oral rehydration salts (ORS sachets) replace lost sodium, potassium, and glucose. Sip small amounts frequently. Adults can use sports drinks, but infants and children should use ORS.
Rest and dietary management
Rest during the acute illness. Resume eating as soon as appetite allows — prolonged fasting is not necessary. Start with bland, easily digestible foods (toast, rice, bananas).
Antiemetics
Ondansetron or metoclopramide reduce vomiting and improve the ability to take oral fluids. Used for moderate to severe vomiting. Domperidone available over the counter for adults.
Self-care and lifestyle
- Stay home from work, school, or food handling for at least 48 hours after the last episode of vomiting or diarrhoea
- Wash hands thoroughly with soap and water (hand sanitiser does not kill norovirus effectively)
- Disinfect surfaces and toilet areas with chlorine-based products
- Wash contaminated clothing and bedding at 60°C
Prevention
Meticulous hand hygiene with soap and water after using the toilet and before eating is the most effective prevention. Rotavirus vaccine (given at 8 and 12 weeks) protects infants against the most common cause of infantile gastroenteritis.