Stomach flu is a viral infection that affects the stomach and small intestine.
It is also called viral gastroenteritis. The illness is usually brief, lasting 1 to 3 days. The illness may develop over a period of hours, or it may suddenly start with stomach cramps, vomiting, or diarrhea. The most important thing to do is to rest the stomach and intestines. You can do this by first eating nothing and drinking only clear liquids.