The flu incubation period is the short period of time following infection in which the flu virus spreads quietly in the body before causing obvious symptoms, states Dennis Clements, M.D., Ph.D., quoted on WebMD. The average incubation period is between one and four days, notes the Centers for Disease Control.
The flu virus becomes contagious during the incubation period. Among healthy adults, the infection can spread to others a day before symptoms appear and up to seven days after they become sick, according to the CDC. Very young children and those with compromised immune systems can be contagious for up to two weeks, says Fox News.