“Stomach flu” is a common name, not a medical diagnosis, and it does not refer to influenza. The illness most people mean is gastroenteritis, an inflammation of the stomach and intestines usually caused by other viruses, such as norovirus.
Influenza is a respiratory illness. The virus infects the nose, throat and lungs. Its typical symptoms include fever, cough, sore throat, a runny or stuffy nose, muscle or body aches, headache and fatigue. They often come on suddenly.
Gastroenteritis looks different. Its main features are nausea, vomiting, watery diarrhea and stomach cramps. Some people also run a mild fever or feel achy, but the illness centers on the gut rather than the airways.
The overlap is where confusion starts. Vomiting and diarrhea can occur with influenza, and they show up more often in children than in adults. Even then, they appear alongside the respiratory symptoms rather than replacing them. A child with flu who vomits will usually also have a fever and a cough.
Norovirus is the best-known cause of gastroenteritis. It spreads easily through contaminated food and water, contaminated surfaces and close contact with a sick person. Outbreaks are common where people share space and meals, such as cruise ships, schools and nursing homes. In the U.K., the NHS calls it the winter vomiting bug.
Other germs cause similar illness. Rotavirus is a leading viral cause in young children, and many countries vaccinate infants against it. Bacteria and parasites can also inflame the stomach and intestines, often through contaminated food or water.
The distinction changes what helps. Flu antivirals such as oseltamivir target the influenza virus specifically, and they do nothing against norovirus or rotavirus. The yearly flu shot is built around influenza strains. It offers no protection against the viruses behind stomach bugs.
Seasonal timing adds to the mix-up. In the Northern Hemisphere, both influenza and norovirus circulate most heavily in the colder months. A household can catch both in the same winter, one after the other.