How Flu Spreads
Person to Person
People with flu can spread it to others. Most experts think that flu viruses spread mainly by droplets made when people with flu cough, sneeze, or talk. These droplets can land in the mouths or noses of people who are nearby (usually within about 6 feet away) or possibly be inhaled into the lungs. Less often, a person might get flu by touching a surface or object that has flu virus on it and then touching their own mouth, nose, or possibly their eyes.
When Are People with Flu Contagious?
Flu viruses can be detected in most infected persons beginning one day before symptoms develop and up to five to seven days after becoming sick. People with flu are most contagious in the first three to four days after their illness begins. However, infants and people with weakened immune systems who are infected with flu viruses may be contagious for longer than seven days.
Symptoms typically begin about two days (but can range from one to four days) after flu viruses infect a person’s respiratory tract. It is theoretically possible that before symptoms begin, an infected person can spread flu viruses to their close contacts. Some people can be infected with flu viruses and have no symptoms but may still be able to spread the virus to their close contacts.