Incubation Period. According to the CDC, although the incubation period can range from one to four days, the typical incubation period is two days. People can begin infecting others about one day before any symptoms of the virus appear, and are contagious for about a week after that.nfluenza is a highly contagious respiratory illness. According to the Centers for Disease Control and Prevention, every year between 5 and 20 percent of people in the United States get the flu.