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World Health Organization (WHO) officials declared a pandemic of H1N1 influenza on Thursday, two months after the first cases of the new flu virus were reported in Mexico. It is the first flu pandemic in 41 years, since the 1968 Hong Kong flu.

The WHO's decision to raise the pandemic alert to Phase 6, the highest level, was based on specific criteria, most significantly that the disease is now widespread, sustaining transmission in more than two regions around the world — including the Americas (Mexico and the U.S., which have so far borne the greatest brunt of the new flu) and Australia, where cases have risen sharply. The latest data show there are nearly 30,000 cases in 74 countries, with 144 confirmed deaths. (See the top 5 swine flu don'ts.)

The...