An influenza pandemic can occur when a new influenza virus subtype emerges and spreads easily among human beings.
Pandemics have been observed for several hundreds years. The best documented pandemics occurred during the 20th century, in 1918 (Spanish flu), 1957 (Asian flu) and 1968 (Hong Kong flu). These varied in severity with an estimated 1-50 million excess deaths during the pandemic.
A new pandemic is considered inevitable but nobody can predict when and where it will emerge, who it will affect or how severe it will be.
The consequences of the next pandemic may be large numbers of cases and deaths globally, as well as disruption of societies and economies due to e.g. staff absenteeism and disruption of trade within and between countries.
WHO uses a series of six phases of pandemic alert to inform the world of the seriousness of the threat and the need to launch progressively more intense preparedness activities.
Based on assessment of all available information and following several expert consultations on influenza A(H1N1), on 29 April 2009, Dr Margaret Chan, WHO's Director-General, raised the current level of influenza pandemic alert from phase 4 to 5.
Pandemic Preparedness Phase 5: a human-to-human spread of the virus with community level infection in at least two countries in one WHO region.
All countries should immediately activate their pandemic preparedness plans and remain on high alert for unusual outbreaks of influenza-like illness and severe pneumonia.