No single virus has the health impact of influenza. Influenza has remained epidemiologically important because it escapes host immune pressure through antigenic variation, is highly contagious, and can cause pneumonia and death in the most susceptible hosts. Viral transmission is most efficient where contact between susceptible hosts is greatest. For humans, this includes institutional settings such as daycare centers, schools, hospitals, and long-term care facilities.
Of the three types of influenza, influenza C is relatively nonvirulent. Influenza B is most virulent in children; its antigenic stability presumably allows the adult population to benefit from acquired immunity. Influenza A is virulent in people of all ages, especially in those at the extremes of age or with immunocompromising disease; the attack rate in persons over 70 years of age is four times that of adults under 40 years of age. A major factor accounting for recurrent influenza A epidemics is change in the virus (antigenic drift and shift) that renders the vaccine less efficacious. Influenza epidemics cost billions of dollars and result in thousands of deaths annually. This discussion will focus on the prevention and treatment of influenza A in the long-term care facility.