Influenza is spread mostly by droplets of respiratory secretions made when people who are infectious cough, sneeze, sing, and talk. The droplets can enter another unprotected person’s mouth or nose or possibly be inhaled in to the lungs. Less often, influenza is spread by touching a surface or object contaminated with influenza virus,
Individuals with influenza are most contagious in the first 3−4 days after their illness begins. Individuals can infect others 1 day before showing symptoms and remain infectious 5−7 days after the onset of symptoms. Those with weakened immune systems may be able to pass on the virus for longer than 7 days.