Why did the World Health Organization call H1N1 a pandemic?
As described above, different strains of the influenza virus are constantly being formed in the wild as a result of mutations in their genetic material. To better understand the risk that a given strain represents to the population at large, the World Health Organization (WHO) has defined six phases of pandemic alert, as you can see on this website: www.who.int/csr/disease/avian_influenza/phase/en/.