When can adults start to pass the flu virus to others?

Adults can start passing the flu virus to others one day before symptoms appear. Influenza is therefore transmissible before a person realizes they are sick.

The flu usually develops after an incubation period of about one to four days. Once symptoms begin, adults can generally shed influenza virus for roughly five to seven days, although contagiousness varies. People are often most contagious during the first three days of illness, when coughing, sneezing, and other respiratory symptoms may be strongest.

This timing explains why influenza can spread quickly through households, workplaces, and schools. An apparently healthy person may transmit the virus through respiratory droplets or small particles released while talking, coughing, or sneezing. Touching contaminated surfaces and then touching the mouth, nose, or eyes can also contribute to spread.

One day is a typical estimate, not a guarantee for every person. Young children, people with weakened immune systems, and those with severe illness may remain contagious longer. Vaccination, staying home while sick, ventilation, hand hygiene, and covering coughs all reduce transmission risk.