If you’ve ever had the misfortune to find yourself stranded by the toilet for 24 to 48 hours purging the contents of your stomach, you may have described the cause as stomach flu. However, actual influenza is primarily a respiratory infection. What keeps you in the bathroom is likely some form of norovirus or rotavirus, which causes gastroenteritis of the stomach and intestines, and usually resolves in one to two days.

The flu shot protects against influenza, which isn’t the same thing as the stomach flu (gastroenteritis). Gastroenteritis is an infection caused by a variety of viruses, including rotaviruses and noroviruses. Although it is often called the stomach flu, gastroenteritis is not caused by influenza viruses. “No,” said Oklahoma Medical Research Foundation immunologist Hal Scofield, M.D. “Although some may call it the “stomach flu”, it actually has nothing to do with the influenza virus.” More accurately called gastroenteritis, contagious stomach illness is most often caused by two viruses, rotavirus or norovirus.