The clinical impact of human respiratory virus infections.

Acute respiratory infections are the most frequent illnesses of the human host. Most infections are caused by viruses and bacteria; the proportion caused by viruses is much greater. The viruses most frequently involved are adenoviruses, influenza viruses, parainfluenza viruses, respiratory syncytial viruses, and rhinoviruses. Acute respiratory infections are more common in young children, have rather specific seasonal occurrences, and some agents are associated with specific respiratory syndromes. Risk factors associated with increased incidence or severity of respiratory infections are occurrence in the very young or the elderly; crowding; being male; inhaled pollutants; anatomic, metabolic, genetic or immunologic disorders; and malnutrition, including vitamin or micronutrient deficiency. Respiratory infections are a much greater problem in developing countries than in developed countries and are the leading causes of death in children under 5 yr of age. The same agents cause infections, and the incidence of total respiratory infections is the same as in the developed countries. The precise causes of increased morbidity and mortality in the developing world are unclear, but crowding, inhaled pollutants, and malnutrition are likely candidates. The interactive role of viruses and bacteria is not clear but may play a role in increased severity of respiratory infections.