A contagious respiratory infection caused by influenza viruses, seasonal flu is transmitted by when infected people sneeze or cough.

People become ill within one to five days from when they are infected. Each year, about 36,000 Americans die as a result on seasonal flu.

Flu pandemics occur when there is a major change in the genetic make up of a virus, which results in a new strain to which people have not been previously exposed.

All humans are susceptible, attack rates are high and mortality rates may be high as well. Pandemic influenza spreads rapidly and can circle the world in only a few months.

The 2009 H1N1 (referred to as "swine flu" early on) is a new influenza virus causing illness in people.

This new virus was first detected in people in the United States in April 2009. This virus is spreading from person-to-person worldwide, probably in much the same way that regular seasonal influenza viruses spread.

There are many avian influenza or "bird flu" viruses and they occur naturally and commonly among domestic poultry and some wild birds, especially waterfowl and shorebirds.

Avian flu viruses mainly infect birds. In low pathogenic strains, disease is mild or absent. In high pathogenic strains, mortality in commercial poultry flocks can be up to 100 percent.

Only in rare instances are these viruses passed to other animals and people — usually through bodily fluids from infected birds.