WHO's Pandemic Flu Alert Scale: What Does It Mean?

Under the World Health Organization's alert system for epidemics, phase 6 is the highest level and means a pandemic — a global epidemic — is under way. Here's a look at the phases.

— Phase 3: An animal or human-animal flu virus causes some infections in people, but with little or no subsequent human-to-human transmission. WHO had its alert at this level as a result of the bird flu outbreaks in Asia since 1997.

— Phase 4: Verified flu transmission among humans able to cause sustained disease outbreaks in a community. WHO declared Phase 4 on April 27 after confirmation of a rash of swine flu cases, including deaths, in Mexico.

— Phase 5: Human-to-human spread of the virus in at least two countries in the same region — a strong signal that a pandemic is imminent. WHO declared Phase 5 on April 29 after a cluster of swine flu cases were confirmed in the United States.

— Phase 6: A global flu pandemic has begun, including community-level outbreaks in at least two world regions. WHO declared Phase 6 on June 11 after increased infections in Europe, Australia, Chile and elsewhere.