Influenza is a highly infectious viral illness. It can cause mild to severe illness and at times lead to death.¹

Most experts think that influenza viruses are spread mainly by droplets made when people with flu cough, sneeze, or talk. These droplets can land in people’s mouths or noses or can be inhaled into the lungs. Putting physical distance between yourself and others can help lower the risk of spreading a respiratory virus.²

Less often, a person might get influenza by touching a surface or object that has flu viruses on it and then touching his or her own mouth or nose.²

Most healthy adults may be able to infect other people with flu beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick. Children may pass the virus for longer than 7 days.²

Symptoms start 1 to 4 days after the virus enters the body, though some people can be infected with the flu virus and show no symptoms. Despite not showing symptoms, people may still spread the flu virus to their close contacts.²

While there are several types of influenza viruses, 2 are predominantly associated with human infection: type A and type B. Certain groups are more likely to get severe illness from influenza, such as adults age 65 years and older, as well as young children, babies, pregnant people, and people with chronic health conditions.^3,4

Type A viruses specifically have different subtypes based on the 2 different proteins they express on their surface: hemagglutinin (H) and neuraminidase (N). For example, influenza A (H3N2) is a type A virus that has H3 and N2 on its surface.⁵ Over time as the virus replicates, small changes in the genes of influenza can accumulate and result in changes to the viruses, which make them antigenically different from previously circulating flu viruses.⁵ This gradual change is referred to as antigenic drift.⁵ As a result of antigenic drift, flu vaccines are updated every season to provide protection against flu viruses most likely to cause illness in the upcoming flu season.^6,7 Of note, occasionally more abrupt changes in the influenza virus can occur as well, known as antigenic shift. Antigenic shift occurs when an influenza virus acquires a new hemagglutinin protein and/or new hemagglutinin and neuraminidase proteins.⁷ Antigenic shifts can result in influenza pandemics if the new flu virus is able to easily spread from person to person and if most or all of the population does not have antibodies to the new shifted flu virus.⁷

The clinical presentation of influenza ranges from asymptomatic infection or a self-limiting upper respiratory tract infection to a severe illness with potentially fatal complications.⁸