Clinical manifestations and consequences of influenza.

The spread of influenza virus through a community typically causes large increases in medical visits for febrile respiratory disease. Increased school absenteeism occurs early in the epidemic, and school children appear to be important for disseminating the virus. Industrial absenteeism, hospitalizations of adults and infants for pneumonia, and deaths due to pneumonia-influenza all tend to peak later in the epidemic. Although influenza infection rates are highest in persons of school age, hospitalizations and deaths occur primarily in infants and in the elderly, particularly among those with pulmonary, cardiovascular, or other debilitating disorders. Influenza viruses can be spread by aerosol or contact. The primary target cells are those of the respiratory epithelium. In healthy adults, the typical influenza syndrome includes fever, cough, and general aches for three to seven days, but lassitude, cough, and evidence of small-airways disease may persist for weeks. Laryngotracheobronchitis, pneumonia, and unexplained fever are prominent manifestations of influenza that lead to hospitalization of young children. Adults are more likely to have complications of bacterial pneumonia and worsening of chronic pulmonary disease or congestive heart failure. Less frequent complications include myositis, various neurologic disorders, and Reye's syndrome. These consequences of influenza clearly justify strenuous efforts at prevention and control.