老人和兒童感染流感病毒可造成嚴重的罹患率和死亡率。 2009 年4 月，A 型H1N1 新型流感（pH1N1）首次在北美被發現，疫情在全世界迅速蔓延。同時，也引起許多嚴重及死亡的案例造成全球的恐慌。新的流感病毒株可能來自病毒基因突變和重配所致，造成不可預知程度的致病性、傳播和大流行。此外，毒性決定因子可提供造成流感病毒的致病性的一些線索並理解造成大流行的原因。因此監控禽流感及豬流感的毒性危險因子，應可及早偵測到具威脅性的病毒株，這些努力將有助於預防及

Infection with influenza viruses can cause severe morbidity and mortality in the elderly and children.In April 2009, a novel H1N1 (pandemic H1N1) influenza virus was first discovered in North America and then spread worldwide rapidly. It also resulted in many severe cases and fatalities which cause global panic. New influenza viral strains may originate from mutations and reassortment of viral genes with unpredictable degreesof pathogenicity, transmissibility, and pandemic potential.Investigations of virulence determinants of influenza A virus may provide the clues of pathogenicity as well as understanding of the factors that cause pandemics. In addition, monitoring the risk markers in avian species as well as swine can help us in early detection of threatening strains which may assist prevention and control of future pandemics. In this review, epidemiology,genomic variations and virulence factors of influenza viruses are discussed.