The typical incubation period for influenza viruses such as influenza A is about one to four days, averaging about two days, explains the Centers for Disease Control and Prevention. Healthy adults become infectious about one day before they develop symptoms and for as long as a week after becoming ill.
Patients with weakened immune systems, and young children can infect others with influenza for more than seven days, notes the CDC. Generally, influenza viruses are transferred from one person to another when an infected person sneezes or coughs near a vulnerable person, known as large-particle respiratory droplet transmission. Such transmissions require close proximity between the infected and vulnerable individuals, since droplets travel only about 3 feet and do not stay airborne. When vulnerable individuals touch contaminated surfaces, they can also transmit influenza viruses.