On Jun. 11, 2009, the Dr. Margaret Chan, Director-General World Health Organization (WHO), declared the world was at the start of 2009 influenza pandemic. The novel H1N1 virus preferentially infected younger people.

Before the H1N1 pandemic in 2009, the influenza A(H1N1) virus had never been identified as a cause of infections in people. Genetic analyses of this virus have shown that it originated from animal influenza viruses and is unrelated to the human seasonal H1N1 viruses that have been in general circulation among people since 1977.

After early reports of influenza outbreaks in North America in April 2009, the new influenza virus spread rapidly around the world. By the time WHO declared a pandemic in June 2009, a total of 74 countries and territories had reported laboratory confirmed infections.

Unlike typical seasonal flu patterns, the new virus caused high levels of summer infections in the northern hemisphere, and then even higher levels of activity during cooler months.

The new virus also led to patterns of death and illness not normally seen in influenza infections.