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

In nearly all areas with large and sustained outbreaks, the majority of cases occurred in people under the age of 25 years. Most cases of severe and fatal infections were in adults between the ages of 30 and 50 years.