Gastroenteritis, also known as infectious diarrhea and gastro, is an inflammation of the gastrointestinal tract including the stomach and intestine. Symptoms may include diarrhea, vomiting, and abdominal pain. Fever, lack of energy, and dehydration may also occur. This typically lasts less than two weeks. It is not related to influenza, even though in the U.S. it is sometimes called the "stomach flu".

Gastroenteritis is usually caused by viruses; however, gut bacteria, parasites, and fungi can also cause gastroenteritis. In children, rotavirus is the most common cause of severe disease. In adults, norovirus and Campylobacter are common causes. Eating improperly prepared food, drinking contaminated water or close contact with a person who is infected can spread the disease. Treatment is generally the same with or without a definitive diagnosis, so testing to confirm is usually not needed.

For young children in impoverished countries, prevention includes hand washing with soap, drinking clean water, breastfeeding babies instead of using formula, and proper disposal of human waste. The rotavirus vaccine is recommended as a prevention for children. Treatment involves getting enough fluids. For mild or moderate cases, this can typically be achieved by drinking oral rehydration solution (a combination of water, salts and sugar). In those who are breastfed, continued breastfeeding is recommended. For more severe cases, intravenous fluids may be needed. Fluids may also be given by a nasogastric tube. Zinc supplementation is recommended in children. Antibiotics are generally not needed. However, antibiotics are recommended for young children with a fever and bloody diarrhea.

In 2015, there were two billion cases of gastroenteritis, resulting in 1.3 million deaths globally. Children and those in the developing world are affected the most. In 2011, there were about 1.7 billion cases, resulting in about 700,000 deaths of children under the age of five. In the developing world, children less than two years of age frequently get six or more infections a year. It is less common in adults, partly due to the development of immunity.