Phase 1 no viruses among animals reported to have caused infections in humans.

Phase 2 animal flu virus is known to have caused infection in humans and is potential pandemic threat.

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.