Gastroenteritis is defined as an acute illness characterized by nausea, emesis, and diarrhea, and associated with malaise, fevers, and anorexia. Gastroenteritis is infectious, with the majority of cases caused by viruses (see viral gastroenteritis). However, particular bacteria (eg, Salmonella, Campylobacter, Shigella, Listeria, enterotoxigenic Escherichia coli, and Clostridioides difficile) can cause gastroenteritis as well.

Many cases of gastroenteritis are self-limited to 24-48 hours of illness. Onset with nausea and emesis resolve with an improving diarrhea. It can be difficult to differentiate between bacterial and viral etiologies. More typically, bacterial causes will be associated with bloody diarrhea and high fevers, although certain viral agents can also present with these symptoms.

Patients of all demographics are susceptible to gastroenteritis, and many infectious causes spread via fecal-oral transmission. Recent travel, contact with infected individuals, food exposure, and recent antibiotic consumption can be important information in delineating a patient's risk of particular bacterial or viral causes of gastroenteritis. Immunocompromised patients are also at increased risk of acquiring gastroenteritis and may be subject to more serious infections, as their immune clearance is impaired.