- Influenza or Seasonal flu is caused by influenza viruses type A and type B which continuously circulate amongst people throughout the world.
Influenza infection peaks during the flu season which is typically from fall to late winter. This corresponds to colder climates and a migration indoors. Cross-infection rates amongst people are higher during this time as people move indoors and use recirculated air and close doors. The flu season tends to be year round in the more tropical climates which do not have a true winter season.

The type A influenza viruses are responsible for pandemics and worse disease compared to the type B influenza.
The current Novel H1N1 Swine flu virus is a type A virus.
The H1N1 notation corresponds to proteins on the surface of the virus. The Swine Flu virus that caused the epidemic of 1918 was also an H1N1 virus, having the same virus surface proteins as the current Swine flu virus.

From 1918 until 1957, the seasonal influenza viruses were predominantly the H1 type of virus. Most people who were born between 1918 and 1957 have been exposed to an H1 form of influenza and should have immunity, antibodies to the Swine Flu type of virus. If they become infected with the Swine flu virus this year, their immune system should recognize the virus with previously made antibodies and they should not get very sick.

People who were born after 1957 have not previously been exposed to the H1 form of the influenza virus.
They do not have immunity or antibodies to this form of influenza and will a more serious infection and worse outcome.
This is the reason for the pandemic; the population does not have immunity to the virus.

The current H1N1 virus will likely to continue to circulate and cause greater disease and mortality for the next several years.

Cristopher Geiler, M.D.
Reference
PLoS Currents: Influenza