到20几天的时间。
https://www.cdc.gov/flu/pandemic-resources/2009-pandemic-timeline.html
April 15
- First human infection with new influenza A H1N1 virus detected in California.
June 11
- The World Health Organization (WHO) declared a pandemic and raised the worldwide pandemic alert level to phase 6, which means the virus was spreading to other parts of the world.
WHO 宣布H1N1 Pandemic, 后来许多国家批评WHO反应过度,这可能也导致WHO这次在宣布Pandemic上比较谨慎。