Viral infections of the upper and lower airways have a major impact on health. Acute respiratory illnesses, caused largely by viruses, are the most common illness experience for otherwise healthy adults and children. Past data from the National Health Interview Survey suggested that such illnesses are experienced at a rate of 85.6 illnesses per 100 persons per year, and account for 54% of all acute conditions exclusive of injuries.1 A total of 44% of these illnesses require medical attention and result in 287 days of restricted activity, 94.4 days lost from work, and 182 days lost from school per 100 persons per year. Estimates from family-based surveillance suggest that approximately one-fourth of these illnesses result in consultation with a physician. Illness rates for all acute respiratory conditions are highest in young children; children below the age of 9 are estimated to experience between 5 and 9 respiratory illnesses per year, whereas adults experience between 3 and 5 such illnesses.2,3
Mortality due to acute viral respiratory infection in otherwise healthy individuals in economically developed countries is relatively rare, with the exceptions of influenza and the recently emergent coronavirus. However, acute respiratory infection is a major cause of childhood mortality in developing countries, and it is estimated that 4.5 million children under 5 years of age die annually from acute respiratory infection.4 Viruses are identified in about 3% to 40% of cases of respiratory disease in this setting and are estimated to play a contributing role in approximately 20% to 30% of deaths. In addition, new and emerging respiratory viruses such as hantavirus, emerging coronaviruses, and transmission of influenza viruses from avian or swine sources to humans pose a continuing threat.
Many of the viruses associated with acute respiratory disease display a significant seasonal variation in incidence. Although the exact seasonal arrival of each virus in the community cannot be predicted with precision, certain generalizations are useful diagnostically and in planning control strategies. For example, influenza and respiratory syncytial virus epidemics both occur predominantly in the winter months, with a peak prevalence in January to March in the northern hemisphere. Parainfluenza virus type 3 (PIV-3) infections show a predominance in the spring, whereas types 1 and 2 (PIV-1 and PIV-2) cause outbreaks in the fall to early winter. Rhinoviruses may be isolated throughout the year, with increases in frequency in the spring and fall. The peak prevalence of enteroviral isolations is in late summer and early fall, whereas adenoviruses are isolated at roughly equal rates throughout the year. The herpes viruses also do not show significant seasonal variation in incidence, except for varicella, which occurs throughout the year but more commonly in late winter and early spring.
The reasons for these seasonal changes are not entirely clear. One mechanism may involve seasonal effects on virus transmissibility either because of more favorable environmental ...