Many people use the term “stomach flu” to describe illnesses with nausea, vomiting or diarrhea, but this isn’t caused by the influenza virus. These symptoms are caused by many different viruses, bacteria or even parasites.

While these symptoms can sometimes be related to the flu — more commonly in kids than adults — these problems are rarely the main symptoms of influenza. The flu is a respiratory disease and not a stomach or intestinal disease. Symptoms usually appear suddenly and include fever, sore throat, cough, headache, chills and muscle aches.

CDC – Misconceptions about Seasonal Flu and Flu Vaccines
CDC – Flu Symptoms & Diagnosis