Have you ever had the "stomach flu?" What you probably had was gastroenteritis - not a type of flu at all. Gastroenteritis is an inflammation of the lining of the intestines caused by a virus, bacteria, or parasites. Viral gastroenteritis is the second most common illness in the U.S. The cause is often a norovirus infection. It spreads through contaminated food or water or by contact with an infected person. The best prevention is frequent hand washing.
Symptoms of gastroenteritis include diarrhea, abdominal pain, vomiting, headache, fever, and chills. Most people recover with no treatment.
The most common problem with gastroenteritis is dehydration. This happens if you do not drink enough fluids to replace what you lose through vomiting and diarrhea. Dehydration is most common in babies, young children, older adults, and people with weak immune systems.
NIH: National Institute of Diabetes and Digestive and Kidney Diseases