Influenza, also known as “the flu,” is a virus that infects the respiratory tract. Although Influenza is not as severe as many viral infections it's almost the worst for viral infections of the respiratory tract. Typically, when someone is infected with influenza they experience fever (usually 100° to 103°F in adults, but even higher in children) and causes a cough, sore throat, runny or stuffy nose, and also headaches, muscle aches, and usually extreme tiredness. There are sometimes other symptoms such as nausea, vomiting and diarrhea but usually only in rare cases with young children. One other note: The term “Stomach flu” isn’t really caused by the influenza virus.