Influenza is a contagious respiratory disease caused by different strains of viruses. In the United States, there is a flu season that begins every fall and ends every spring. The type of flu people get during this season is called seasonal flu. Flu viruses spread from person to person when people who are infected cough or sneeze. Adults may be able to infect others 1 day before getting symptoms and as long as 5 days after getting sick.Get specific guidance for H1N1

A pandemic is a global disease outbreak. An influenza pandemic occurs when a new influenza virus emerges for which there is little or no immunity in the human population, begins to cause serious illness and then spreads easily person-to-person worldwide.

There have been three instances of a pandemic flu epidemic in the 1900s

• 1918 influenza pandemic caused at least 675,000 U.S. deaths and up to 50 million deaths worldwide
• 1957 influenza pandemic caused at least 70,000 U.S. deaths and 1-2 million deaths worldwide
• 1968 influenza pandemic caused about 34,000 U.S. deaths and 700,000 deaths worldwide