Influenza: quantifying morbidity and mortality.

Because of their dramatic impact, morbidity and mortality associated with influenza have been recognized since at least the time of Elizabeth I of England. Excess mortality has been documented since 1889, and the infamous 1918 outbreak confirmed that influenza was truly one of the last major plagues. Despite the clear recognition of large clusters of influenza activity, it is still difficult to quantify precisely the total impact of influenza morbidity and mortality, since laboratory confirmation is required for exact diagnosis. Many methods have been developed to provide estimates of the mortality associated with influenza. These methods are usually predicated on establishing expected baseline rates of mortality. Deaths in excess of these rates are then calculated--and attributed to the circulating influenza virus. In this way, groups at high risk of mortality have been defined as the elderly and those with chronic conditions. Special-risk groups, such as those in institutions, have also been identified. The quantification of morbidity has required different approaches, and here community and family studies have made major contributions. In contrast to mortality, morbidity is most pronounced in children and young adults, and the diseases, although self-limited, are often quite severe. Although the size of the outbreaks varies, influenza infection can be documented annually. Thus, each year must be considered an influenza year.