The first case of Influenza A (H1N1) occurred on Mar. 2009, when human infection with swine flu epidemic broke out in Mexico. The influenza quickly spreaded throughout the world. The WHO referred it initially as human infection with swine flu, but later nominated it as Influenza A (H1N1). On June 11, 2009, WHO declared to raise its epidemic alarming level to grade VI, signaling its prevailing worldwide. The clinical symptoms of Influenza A (H1N1) were similar to those of regular human seasonal influenza. However, Mexico saw its high mortality rate, quicker spreading rate and susceptibility in general population.

On Apr. 30, 2009, Influenza A (H1N1) was listed as Class B infectious diseases and the frontier health and quarantine infectious diseases in China. It has been managed as the Class A infectious diseases since then [1] (Table 3.1).