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. These strains are different from the original virus but retain some of its characteristics. 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 every year.

The virus is spread from person to person through:

• Direct contact, such as shaking hands.
• Small droplets that form when a person sneezes or coughs.
• 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?

People who have the flu are most likely to pass it to someone else from 1 day before to 5 days after symptoms develop. Children may be infectious for up to 6 days before symptoms develop.

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.