Governments and health officials around the world continued to take steps Tuesday against the outbreak of swine flu that has killed scores of people in Mexico and spread to the U.S., Europe and possibly Asia.
In Phase 1 no viruses among animals reported to have caused infections in humans.
In Phase 2 animal flu virus is known to have caused infection in humans and is potential pandemic threat.
In Phase 3, animal or human-animal flu virus has caused small number of cases in people, but not sufficient for community-level outbreaks.
Phase 4 is characterized by verified human-to-human transmission of animal or human-animal flu virus, indicating greater but not inevitable risk of pandemic.
Phase 5 means human-to-human spread of virus is confirmed in two countries in one WHO region and pandemic is feared imminent.
Phase 6 characterized by community-level outbreaks in at least one other country in second WHO region. This phase indicates global pandemic is under way.