This chapter provides a brief overview of the influenza virus and past pandemics and then focuses on understanding the risks to healthcare workers.

Influenza is a serious respiratory illness caused by infection with influenza type A or type B virus. Since the beginning of the twentieth century, only the influenza A virus has been associated with infection in humans. Cases of influenza peak during the winter months in each hemisphere. In addition to seasonal occurrences of influenza, outbreaks may result in a global pandemic. For seasonal influenza, the risk of serious illness and death is highest among persons over the age of 65 years, children under 2 years of age, and persons who have medical conditions that place them at increased risk of developing complications from influenza. Each year in the United States more than 35,000 deaths and 200,000 hospitalizations result from influenza and its complications, with most of the excess mortality in persons 65 years and older, often from pneumonia (Lewis, 2006; CDC, 2007). Vaccines and antiviral medications have been developed to prevent or mitigate the disease, although major challenges remain, particularly in determining the appropriate virus subtype to target. In a review of nine studies, Brankston and colleagues (2007) note that infections in individuals exposed to influenza ranged from 33 to 55 percent in unvaccinated and 0 to 37 percent in vaccinated cohorts.