How WHO measures a pandemic

- Phase 1. There are no viruses circulating in animals that have been reported to cause infections in humans.

- Phase 2. An animal flu virus has caused infections in humans in the past and is considered to be a potential pandemic threat.

- Phase 3. An animal or mixed animal-human virus has caused occasional cases or small clusters of disease, but the virus does not spread easily. The world is currently in phase 3, with H5N1 bird flu viruses sporadically infecting humans and occasionally spreading from human to human.

- Phase 4. The new virus can cause sustained outbreaks and is adapting itself to human spread.

- Phase 5. The virus has spread into at least two countries and is causing even bigger outbreaks.

- Phase 6. More outbreaks in at least two regions of the world; the pandemic is under way.

If the World Health Organization raises its pandemic alert level to 4 or 5, that signals that the swine flu virus is becoming increasingly adept at spreading between humans. That would signal to governments they should ready their pandemic preparedness plans and increase detection systems for potential cases.

With an elevated pandemic alert level, WHO might also issue travel advisories, warning against nonessential travel to regions battling outbreaks, trade restrictions, the cancellation of public events or border closures.

During the SARS outbreak in 2003, WHO travel advisories drastically slashed travel to affected regions, curtailing the outbreak.

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