Gastroenteritis (Stomach bug or Stomach flu) Gastroenteritis, often called a stomach bug or stomach flu, is irritation or inflammation of the stomach and intestines. It commonly causes nausea, vomiting, diarrhea, stomach cramps, and sometimes fever. Despite the nickname “stomach flu,” gastroenteritis is not the same as influenza, which is a respiratory virus. A stomach bug usually affects the digestive system. What is gastroenteritis? Gastroenteritis can be caused by viruses,