From Virology Blog, How does the flu virus spread? Read about it in “Influenza virus transmission.”
Influenza virus may be transmitted among humans in three ways: (1) by direct contact with infected individuals; (2) by contact with contaminated objects (called fomites, such as toys, doorknobs); and (3) by inhalation of virus-laden aerosols.
Read on for how to prevent transmission.
As I read this, I realized that the paper masks are not especially to keep people safe, but to keep them from spreading aerosols as they speak or cough. Having a handkerchief to whip out before a sneeze is going to be very good manners.