Influenza is a viral infection of the respiratory tract that causes coughing, breathing difficulty, fever, headache, muscle aches and weakness. The virus is spread from person to person by inhaling infected droplets from the air.
Review Date 8/13/2020
Updated by: Linda J. Vorvick, MD, Clinical Associate Professor, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.