AFTER its debut in late March， A/H1N1 flu soon spread from Mexico and the ，U.S. to more than 30 coun-tries and regions worldwide， includ-ing China's mainland and Hong Kong，causing global alarm and prompting a flurry of precautions. By April 27 the World Health Organization had raised its pandemic alert level from Phase 3 to 4， and only two days later， to the second place register on the scale.