Phase 1: A virus in animals has caused no known infections in humans.
Phase 2: An animal flu virus has caused infection in humans.Phase 3: Sporadic cases or small clusters of disease occur in humans. Human-to-human transmission, if any, is insufficient to cause community-level outbreaks.Phase 4: The risk for a pandemic is greatly increased but not certain.Phase 5: Spread of disease between humans is occurring in more than one country of one WHO region.Phase 6: Community-level outbreaks are in at least one additional country in a different WHO region from phase 5. A global pandemic is under way.How many people die from a pandemic depends upon:
The number of people who become infectedThe severity of disease caused by the virus (its virulence)The vulnerability of affected populationsThe effectiveness of preventive steps
