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A pandemic flu is different from a seasonal outbreak of the disease. An influenza pandemic spreads easily from person to person worldwide, and past pandemics have led to high levels of illness, death, social disruption, and economic loss. Scientists believe that it is only a matter of time until another pandemic occurs.
A pandemic flu is a global outbreak of disease that occurs when a new influenza A virus subtype appears, or "emerges," in the human population, causes serious illness, and then spreads easily from person to person worldwide.
Pandemics are different from seasonal outbreaks or "epidemics" of influenza (the flu). Influenza viruses that already circulate among people cause seasonal outbreaks, while an outbreak of pandemic flu is caused by:
- New subtypes
- Subtypes that have never circulated among people
- Subtypes that have not circulated among people for a long time.
Past influenza pandemics have led to high levels of illness, death, social disruption, and economic loss.
There are many different subtypes of influenza or "flu" viruses. The subtypes differ based upon certain proteins on the surface of the virus, which include the hemagglutinin (HA) protein and the neuraminidase (NA) protein.
Pandemic flu viruses emerge as a result of a process called "antigenic shift," which causes an abrupt or sudden major change in influenza A viruses. These changes are caused by new combinations of the HA and/or NA proteins on the surface of the flu virus. Such changes result in a new influenza A virus subtype. The appearance of a new influenza A virus subtype is the first step toward a pandemic flu (see Pandemic Stages). However, to cause a pandemic flu, the new virus subtype must also have the capacity to spread easily from person to person. Once a new pandemic flu virus emerges and spreads, it usually becomes established among people and moves around or "circulates" for many years as seasonal epidemics of influenza.