Influenza gets blamed for a lot of ailments that it doesn't actually cause; for example, the "stomach flu." There really isn't such a thing as stomach flu. The flu doesn't involve vomiting or any other gastrointestinal distress -- it's a respiratory illness that's caused when you come in contact with a human influenza virus. Noroviruses, however, do cause stomach flu symptoms; nausea, vomiting, diarrhea and abdominal pain are all classic norovirus complaints -- and you may also have a fever, headache or general body aches (which might make you think you have influenza). Norwalk, Snow Mountain and Hawaii noroviruses account for an estimated 90 percent of all cases of so-called stomach flu worldwide -- and about 20 million infections annually among Americans [source: Nordqvist].