Answer: The stomach flu is viral gastroenteritis
The stomach flu is actually viral gastroenteritis. Gastroenteritis1 is the inflammation of the stomach and intestines that causes vomiting and diarrhea. When we refer to the "stomach flu", we are talking about gastroenteritis that is caused by an assortment of different viruses including norovirus and rotavirus. The “stomach flu” is not really a flu at all and has no relation to the influenza viruses. The annual flu shot does not protect you from the stomach flu. Influenza (the real flu) can cause vomiting and diarrhea, though. So, it can be very confusing. Viral gastroenteritis is commonly referred to as "stomach flu", "stomach virus", "stomach bug", "24 hour bug", or "GI bug", "GI virus", "winter vomiting disease". Some people get very angry that I use the nickname the "stomach flu". There just isn't a good collective term for all of the throwing up illnesses that I hate. The instead of the "flu" in stomach flu referring to influenza, think of it referring to the sentence "the contents of my stomach flew out both ends." There are other causes of gastroenteritis including bacterial and parasitic infections. Food poisoning can also cause gastroenteritis. This website discusses all of these causes so you can learn to avoid them. It is very important to clean with a product that actually kills norovirus when a stomach bug is in your home such as the Clorox® Hydrogen Peroxide Spray
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--Annie Pryor, Ph.D.