The sneezes and coughs of an infected person scatter droplets containing the influenza virus. The droplets are relatively large and do not travel far, but the virus can get directly into the respiratory tract of people who are just one to two meters away from the infected person, causing them to also become infected. The virus can also enter the body directly through mucus membranes in the eyes and elsewhere.

The virus gets into the body through mucus membranes when people unconsciously bring their hands with the virus on them, into contact with their eyes, nose, or mouth. Since the virus can survive for a long time in a dry environment, it spreads when it gets onto things such as telephones, doorknobs, utensils, and handrails on trains and buses, causing infections when other people touch those later.