The incubation period for the flu is between one to four days and averages two days, according to the Centers for Disease Control and Prevention. Healthy adults can infect others one day before developing symptoms and up to five to seven days after becoming sick.
Some people, such as young children and people with weakened immune systems, may be able to infect others for a longer time, CDC states. The flu virus is spread mainly through respiratory droplets when an infected person coughs or sneezes near a susceptible person. The flu can also be spread through contact with surfaces contaminated with respiratory droplets.