The incubation period for influenza ranges from one to four days, while the average is two days, notes the Centers for Disease Control and Prevention. Most adults have the ability to infect others as early as a full day before presenting symptoms and up to a week after getting sick.
People who have compromised immune systems and young children have the ability to infect others for a longer period of time, reports the Centers for Disease Control and Prevention. Transmission most often takes place when a sick person sneezes or coughs near someone susceptible to infection, although it's also possible through contact with surfaces that harbor droplets from the cough or sneeze. The data supporting airborne transmission of the disease through small-particle residue is inconclusive, as of 2015.