The flu is caused by the
influenza virus. Doctors classify the virus as
influenza type A and type B, each of which includes several subtypes or
strains. Type A is usually responsible for the annual outbreaks that typically
occur in the late fall and early winter.
The influenza virus
changes often, so having flu caused by one strain does not give you full
immunity to other strains.
Widespread outbreaks of the flu usually follow
significant changes (called antigenic shifts) in the virus and occur about
every 10 years. People who get the flu tend to become much sicker when a shift
in the flu virus occurs.
Minor changes in the virus (called antigenic drifts) occur nearly
The virus is spread from person to person through:
Direct contact, such as shaking hands.
Small droplets that form when a person sneezes or
Contact with objects such as handkerchiefs that have been
in contact with fluids from an infected person's nose or throat.
When are you contagious?
If you are infected with the flu, you are most likely to pass it to someone else from 1 day before symptoms start and up to 7 days after
symptoms develop. Children may be infectious for longer than 7 days after symptoms start.
Symptoms usually develop 1 to 4 days after you are
infected. Because symptoms may not develop for a couple of days, you may pass
the flu to someone before you know you have it.