Gastroenteritis (also known as gastric flu or stomach flu, although unrelated to influenza) is inflammation of the gastrointestinal tract, involving both the stomach and the small intestine and resulting in acute diarrhea. It can be transferred by contact with contaminated food and water. The inflammation is caused most often by an infection from certain viruses or less often by bacteria, their toxins, parasites, or an adverse reaction to something in the diet or medication. Worldwide, inadequate treatment of gastroenteritis kills 5 to 8 million people per year, and is a leading cause of death among infants and children under 5.

At least 50% of cases of gastroenteritis due to foodborne illness are caused by norovirus. Another 20% of cases, and the majority of severe cases in children, are due to rotavirus. Other significant viral agents include adenovirus and astrovirus.

Different species of bacteria can cause gastroenteritis, including Salmonella, Shigella, Staphylococcus, Campylobacter jejuni, Clostridium, Escherichia coli, Yersinia, Vibrio cholerae, and others. Some sources of the infection are improperly prepared food, reheated meat dishes, seafood, dairy, and bakery products. Each organism causes slightly different symptoms but all result in diarrhea. Colitis, inflammation of the large intestine, may also be present.

Risk factors include consumption of improperly prepared foods or contaminated water and travel or residence in areas of poor sanitation. It is also common for river swimmers to become infected during times of rain as a result of contaminated runoff water.