Influenza (flu) is a respiratory tract infection caused by the influenza virus, primarily affecting the nose, throat, and bronchi, and rarely spreading to the lungs. This virus spreads rapidly through coughing, sneezing, close contact, kissing, and shaking hands. Insufficient hand washing, in particular, increases the risk of viral transmission. The likelihood of transmission is higher in enclosed spaces, as millions of viruses are dispersed into the air when an infected person coughs or sneezes, and are then inhaled by healthy individuals. Individuals without immunity to the influenza virus typically contract the flu within 1 to 4 days after exposure to the virus.