supported by The Research Grants Council of The HongKong SAR Government (7488/05M) ;the Research Fund for the Control of Infectious Diseases of the Health, Welfare and Food Bureauof the Hong Kong SAR Government, the Li Ka Shing Foundation;the Providence Foundation in memory of The late Dr. Lui Hac Minh;grants from The National Natural Science Foundation of China(30571674, 30771988)~~