The flu, short for influenza, is a highly contagious respiratory infection
caused by the influenza virus. The flu can cause mild illnesses for some people.
For older people, especially those with a chronic condition, the flu
can be very serious, even life-threatening.
The flu is easily transmitted from person to person. You can catch the flu when someone who is infected coughs, talks, or sneezes near you. Or you can catch it if you touch a recently contaminated surface or object that has the flu virus on it and then touch your nose, eyes, or mouth.Learn about flu symptoms >
In the U.S., flu season occurs in the fall and winter. While influenza viruses circulate year-round, flu activity typically peaks between December and February, but it can last up until May.
The Centers for Disease Control and Prevention (CDC) collects and analyzes information on flu activity year-round in the U.S. It posts a weekly report that includes information on what viruses are circulating, as well as details on flu-related hospitalizations and deaths.
According to the CDC, there are 2 types of influenza viruses that cause seasonal flu epidemics: A and B.
Subtypes of type A have caused some of the deadliest global flu pandemics in history.
H1N1, which caused swine flu in 2009
H3N2, which caused Hong Kong flu in 1968
Type B viruses can cause outbreaks of seasonal flu.