Official reports say there have been 28,000 cases globally and 141 deaths and figures are rising daily.
WHO spokesman Gregory Hartl said it had been expecting something more like the deadlier bird flu.
"It was believed that the next pandemic would be something like H5N1 bird flu, where you were seeing really high death rates, and so there were people who believed we might be in a kind of apocalyptic situation and what we're really seeing now with H1N1 is that in most cases the disease is self-limiting," he told the BBC.
The WHO will have to manage the global anxiety the declaration of a pandemic will generate, our correspondent says.
In nature, influenza viruses circulate continuously among animals, especially birds. Even though such viruses might theoretically develop into pandemic viruses, in Phase 1 no viruses circulating among animals have been reported to cause infections in humans.
In Phase 2 an animal influenza virus circulating among domesticated or wild animals is known to have caused infection in humans, and is therefore considered a potential pandemic threat.
In Phase 3, an animal or human-animal influenza reassortant virus has caused sporadic cases or small clusters of disease in people, but has not resulted in human-to-human transmission sufficient to sustain community-level outbreaks. Limited human-to-human transmission may occur under some circumstances, for example, when there is close contact between an infected person and an unprotected caregiver. However, limited transmission under such restricted circumstances does not indicate that the virus has gained the level of transmissibility among humans necessary to cause a pandemic.
Phase 4 is characterized by verified human-to-human transmission of an animal or human-animal influenza reassortant virus able to cause “community-level outbreaks.” The ability to cause sustained disease outbreaks in a community marks a significant upwards shift in the risk for a pandemic. Any country that suspects or has verified such an event should urgently consult with WHO so that the situation can be jointly assessed and a decision made by the affected country if implementation of a rapid pandemic containment operation is warranted. Phase 4 indicates a significant increase in risk of a pandemic but does not necessarily mean that a pandemic is a forgone conclusion.
Phase 5 is characterized by human-to-human spread of the virus into at least two countries in one WHO region. While most countries will not be affected at this stage, the declaration of Phase 5 is a strong signal that a pandemic is imminent and that the time to finalize the organization, communication, and implementation of the planned mitigation measures is short.
Phase 6, the pandemic phase, is characterized by community level outbreaks in at least one other country in a different WHO region in addition to the criteria defined in Phase 5. Designation of this phase will indicate that a global pandemic is under way.
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They were hunting for mosquitoes - or more specifically, fetid swimming pools that harbor the insects and potentially the West Nile virus they can carry.
As the foreclosure crisis has escalated, so has the number of abandoned pools. A million mosquitoes can breed in a single stagnant pool. Now finding and fixing them has taken on a new level of urgency.
Last year, mosquito districts hired it to fly over almost every county in California. It covered 3,500 square miles - about one-third of the state's urban area - and "harvested" 27,000 algae-ridden pools
"Countries must deal with the arrival of the NEW disease"
The World Health Organization declared a swine flu pandemic Thursday — the first global flu epidemic in 41 years — as infections in the United States, Europe, Australia, South America and elsewhere climbed to nearly 30,000 cases.
The long-awaited pandemic announcement is scientific confirmation that a new flu virus has emerged and is quickly circling the globe. WHO will now ask drugmakers to speed up production of a swine flu vaccine. The declaration will also prompt governments to devote more money toward efforts to contain the virus.
WHO chief Dr. Margaret Chan made the announcement Thursday after the U.N. agency held an emergency meeting with flu experts. Chan said she was moving the world to phase 6 — the agency's highest alert level — which means a pandemic, or global epidemic, is under way.
"The world is moving into the early days of its first influenza pandemic in the 21st century," Chan told reporters. "The (swine flu) virus is now unstoppable."
