Viral diseases exert their major impact through morbidity, impairment of personal health, loss of time at work and school, and cost of medical care. Relatively few of the known viruses cause a significant number of deaths; influenza, childhood viral pneumonia, and hepatitis are the only viral diseases causing more than 1,000 deaths per year. Data based on the National Health Interview Survey of the National Center for Health Statistics show that the common cold annually causes 35.6 acute illnesses per 100 persons. The data reported by the National Therapeutic Disease Index (on the basis of visits by patients to a sample of 1,500 private physicians) show that influenza and other acute respiratory conditions account for about one-third of all visits to physicians. Nearly all viruses first infect humans in infancy and childhood, with a relatively low fatality rate but with frequent episodes of illness.