The World Health Organization declared the 2009-2010 flu season a pandemic because the novel H1N1 flu (swine flu) had occurred in multiple countries around the world and human infection was widespread. The classification does not reflect the severity of individual infections.
Flu pandemics typically occur when a virus mutates into something very different from what people have been exposed to in the past. The first H1N1 pandemic occurred in 1918 and it killed millions of people. Luckily, the 2009 pandemic was much milder.
Dec. 04, 2010
- Thorner AR. Epidemiology of pandemic H1N1 influenza ('swine influenza'). http://www.uptodate.com/home/index.html. Accessed Nov. 12, 2010.
- Morens DM, et al. The persistent legacy of the 1918 influenza virus. New England Journal of Medicine. 2009;361:225.
- Fiore AF, et al. Prevention and control of influenza with vaccines: Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2010. MMWR Early Release. 2010;59:1. http://www.cdc.gov/mmwr/pdf/rr/rr59e0729.pdf. Accessed Aug. 2, 2010.