Influenza viruses cause flu, a respiratory illness that is contagious and affects the nose, throat, and at times, the lungs.
The symptoms of flu include fever or feeling feverish/chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, and fatigue (tiredness).
The main mode of transmission for many viruses, including the flu, is through small droplets that are expelled when infected individuals cough, sneeze, or talk.
Each flu season, the percentage of the U.S. population that becomes ill from the flu can range from 3 percent to 11 percent.
It can be challenging to differentiate the flu from other viral or bacterial respiratory illnesses based solely on symptoms.