Influenza

Influenza is a seasonally occurring viral illness that can result in sporadic, epidemic, and pandemic morbidity and mortality. Disease in humans is caused predominantly by the influenza A and B viruses, and effective vaccines are available for the prevention and primary control of illness. Antigenic drift of viral surface antigen composition causes small changes in circulating viruses, requiring annual immunization with vaccine matched to viral isolates obtained through national and global surveillance. Antigenic shift causes larger-scale changes in viral surface antigen characteristics and pathogenicity, which can result in pandemic infection. Previous pandemics have caused between 1. million and 100. million deaths worldwide. The threat of impending pandemic influenza and its associated public health burden justify both preparedness and interventions to control influenza.