Influenza, or the flu, is a respiratory infection caused by the influenza virus that primarily affects the nose, throat, and bronchi, and less commonly, the lungs. The influenza virus spreads easily through coughing, sneezing, and close contact such as kissing and shaking hands; its spread is facilitated by infrequent handwashing.

The flu spreads much more easily in enclosed spaces. When an infected person coughs or sneezes, millions of viruses are released into the air. Healthy individuals then contract the virus through respiration. Those without immunity to the influenza virus typically develop the flu within 1-4 days of exposure.