Definition of H1N1 (swine) influenza:

The H1N1 virus (swine flu) was a new flu virus strain that caused a worldwide pandemic in humans from June 2009 to August 2010.

The Centers for Disease Control and Prevention now call the virus 2009 H1N1.

Causes, incidence, and risk factors:

Earlier forms of the H1N1 virus were found in pigs. Over time, the virus changed (mutated) and infected humans. Because H1N1 was a new virus in humans in 2009, it spread quickly around the world. The largest number of H1N1 flu cases occurred in people ages 5 - 24. Fewer cases were reported in people older than age 64.

During the 2010-2011 flu season, H1N1 virus (swine flu) did not cause widespread infections as it had in 2009-2000. The 2010 seasonal flu vaccine also protected against swine flu, and a separate vaccine was not needed. Most likely, the 2011 seasonal flu vaccine will also protect against swine flu.

Any flu virus can spread from person to person when:

• Someone with the flu coughs or sneezes into air that others breathe in.
• Someone touches a doorknob, desk, computer, or counter with the H1N1 germs on it and then touches their mouth, eyes, or nose.
• Someone touches mucus while taking care of a child or adult who is ill with the H1N1 flu virus.

You CANNOT get H1N1 flu virus from eating pork or any other food, drinking water, swimming in pools, or using hot tubs or saunas.

For more information about the symptoms, treatment, and prevention of the flu, see also: Influenza vaccine.