Most human-specific strains of influenza spread from person to person via airborne droplets or from contact with contaminated surfaces, according to the Centers for Disease Control and Prevention. The virus' contagious phase begins a day before symptoms manifest and persists for up to a week after onset.
Influenza enters the body through the mouth and nose, commandeers the cells of the throat or lungs, and uses their internal chemical machinery to produce more viruses. Infected people are able to spread the virus to others within a few days of infection and may be contagious before any symptoms of flu are apparent, explains the CDC. During the contagious period, which lasts five to seven days after the onset of symptoms, the virus is carried out of the body in droplets of saliva and mucus that are expelled when patients sneeze, cough or talk. It is also possible to spread influenza by handling linens, clothing and other items that have been contaminated with infectious material and touching the face.
Preventing the spread of influenza requires that sick people avoid contact with others, and that uninfected people wash their hands frequently, according to the CDC. Children are especially vulnerable to influenza and may remain contagious longer than adults.