Influenza is an illness from a virus that causes fever, coughing, muscle aches, and other symptoms that we refer to as flu. A flu pandemic occurs when a new flu virus spreads to people all over the world.

Pandemic flu is not the same as seasonal flu. Every year people get sick from seasonal flu. Getting vaccinated (flu shot or nasal spray) can protect you from seasonal flu. Pandemic flu might make you sicker than seasonal flu. It may also spread more easily from person to person. Learn more about seasonal flu vs. pandemic flu.

H1N1 flu (formerly known as swine flu) was a flu pandemic. On August 10, 2010, the World Health Organization (WHO) announced that the world is no longer in an influenza pandemic*†. Although we are in the post-pandemic period, the H1N1 virus is still circulating.

History of Flu Pandemics

The threat of a pandemic is not new. It is estimated that flu pandemics have been affecting the world for the last four centuries. There were three pandemic flu outbreaks in the 20th century:

• In 1918 the world coped with the Spanish Flu, a pandemic that caused 20 to 40 percent of the people in the world to become ill and caused 50 million deaths. At least 675,000 deaths were in the United States.
• In 1957 the Asian Flu killed 69,800 people in the United States alone, and one to two million worldwide.
• In 1968, the Hong Kong Flu caused 33,800 deaths in the U.S. and 700,000 deaths worldwide.

Learn more about 20th century flu pandemics*.

Pandemic Flu Phases

The World Health Organization (WHO) has defined the phases of a pandemic to provide a global framework to aid countries in pandemic preparedness and response planning.

Pandemics can be either mild or severe in the illness and death they cause, and the severity of a pandemic can change over the course of that pandemic. Learn about WHO Pandemic Influenza Phases*†.

Potential Impact of Pandemic Flu

Waves of pandemic flu may occur in communities for 6-8 weeks, then occur again months later. The country and the world could be affected for 1 to 2 years.

During a pandemic, 3 (or more) in every 10 people could become ill. During a peak, 4 out of every 10 workers may miss work due to their own illness, caring for ill family, or other reasons.

A pandemic could change our communities in many ways. For example:

• Schools may be dismissed for weeks.
• Public gatherings (sports, concerts, movies) may be cancelled.
• Public transportation could be limited.
• Businesses and public services may close or limit hours.
• Some people with flu symptoms may be asked to stay home or away from others while ill.
• Those exposed to illness may also be asked to stay away from others until it is clear any threat of illness has passed.