Phase 1: No infections in humans are being caused by viruses circulating in animals.

Phase 2: Animal flu virus causes infection in humans, and is a potential pandemic threat.

Phase 3: Flu causes sporadic cases in people, but no significant human-to-human transmission.

Phase 4: Human-to-human transmission and community-level outbreaks.

Phase 5: Human-to-human transmission in at least two countries. Strong signal pandemic imminent.

Phase 6: Virus spreads to another country in a different region. Global pandemic under way

Post-peak: Pandemic activity appears to be decreasing though second wave possible.
Post-pandemic: activity returns to normal, seasonal flu levels

รูปจาก BBC News  WHO raises pandemic alert level