Gastroenteritis is a condition that causes irritation and inflammation of the stomach and intestines (the gastrointestinal tract). An infection may be caused by bacteria or parasites in spoiled food or unclean water. Some foods may irritate your stomach and cause gastroenteritis. Lactose intolerance to dairy products is one example.
Many people who experience the vomiting and diarrhea that develop from these types of infections or irritations think they have "food poisoning," which they may, or call it "stomach flu," although influenza has nothing to do with it.-www.emedicinehealth.com
There goes the steak...