The World Health Organization's 194 nations must now prepare for the new flu strain, which is expected to infect one-third of the population.
Margaret Chan, director general of the organization said in an afternoon news conference in Geneva, "We anticipate this action will raise many questions and that often these questions do not have simple answers."
She is advising the world's makers of flu vaccines "quickly to prepare commercial-scale pandemic vaccine of this H1N1 virus."
Worldwide there have been 28,774 cases in 74 countries, and 144 deaths.
Currently the outbreaks in Australia, Chile and Japan, as well as concern that the virus is far more widespread in Europe than officials there have admitted, made the call by Dr. Chan to move the level of danger to Phase 6.
The big question is when could medical providers expect the arrival of vaccines against the virus. Manufacturers might be able to provide the first doses by September with an anticipated 2 billion doses over the next year.
Dr. Chan says she expects the virus to sketch a "bleaker" picture in poorer countries that will not have the financial ability to purchase the vaccines that have already been contracted by industrialized nations.
For now the WHO is calling this a "moderate" pandemic, deliberately avoiding the word "mild' it had previously used to label the illness.
