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Background to pandemic influenza

Influenza A (H1N1) is a highly contagious acute respiratory disease. The disease circulates in humans and is also common in various animal species, prominent among which are pigs and birds. Many different sub-types of influenza exist and pigs can be infected with more than one virus type at a time, which can allow the genes from these viruses to mix. The resulting “reassortant” virus and other influenza viruses can acquire the capacity to cross the species barrier and cause disease in humans. Humans often will have no immunity to this virus. When, in addition, a virus becomes easily transmissible between humans, then a pandemic - the global spread of an epidemic - can occur.

In April 2009, a new subtype of influenza A (H1N1) not previously detected either in swine or in humans, containing a mix of swine, avian and human virus genes started to cause influenza in humans in Mexico. This new influenza A (H1N1) spread rapidly to many countries and on 11 June 2009, the Director-General of WHO, after consulting the IHR Emergency Committee, the most-affected Member States and other public health experts, declared a pandemic.