What one characteristic will most likely lead to a disease outbreak being classified as a pandemic?

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 populations The effectiveness of preventive steps