Considering how common illness is, how tremendous the spiritual change that it brings, how astonishing, when the lights of health go down, the undiscovered countries that are then disclosed, what wastes and deserts of the soul a slight attack of influenza brings to view… —Virginia Woolf, “On Being Ill”
Respiratory disease accounts for an estimated 75% to 80% of all acute morbidity in the United States, and most of these illnesses (approximately 80%) are viral infections. Although a majority of the episodes may not require medical attention, the overall average is three to four illnesses per year per person. Although the incidence varies inversely with age (ie, greater among younger children than healthy young adults), the morbidity is significantly higher in elderly population. Seasonality is also a feature; incidence is lowest in the summer months and highest in the winter.
Respiratory viruses represented by diverse viral agents from different virus families
The viruses that are major causes of acute respiratory disease (ARD) include influenza viruses, parainfluenza viruses, rhinoviruses, adenoviruses, respiratory syncytial virus (RSV), human metapneumovirus (hMPV), and respiratory coronaviruses. Recently, bocaviruses (member of the parvovirus group) have also been associated with acute respiratory illness. Reoviruses are of questionable importance, but are also considered. Others, such as enterovirus and measles virus, can also cause respiratory symptoms but are discussed in other chapters.
In addition to the ability to cause a variety of ARD syndromes, this somewhat heterogeneous group of viruses shares a relatively short incubation period (1-4 days) and a person-to-person mode of spread. Transmission is direct, by infective droplet nuclei, or indirect, by hand transfer of contaminated secretions to nasal or conjunctival epithelium. All these agents are associated with an increased risk of bacterial superinfection of the damaged tissue of the respiratory tract, and all have a worldwide distribution.
Short incubation period
Transmission by droplet nuclei or hand transfer of contaminated secretions
INFLUENZA VIRUS GROUP CHARACTERISTICS
Influenza viruses are members of the orthomyxovirus group or family, which are enveloped, pleomorphic, single-stranded negative-sense segmented RNA viruses. They are classified into three major types, A, B, and C, on the basis of antigenic differences in their ribonucleoprotein (NP) and matrix (M) protein antigens. Influenza A viruses are the most extensively studied because of their predominance in epidemics, and much of the following discussion is based on knowledge of this type. They generally cause more severe disease and more extensive epidemics than the other types; naturally infect a wide variety of species, including mammals and birds; and have a great tendency to undergo significant antigenic changes (Table 9–1). Influenza B viruses are more antigenically stable, are known to infect humans and seals, and usually occur in more localized outbreaks. Influenza C viruses appear to be relatively minor causes of disease, affecting humans and pigs.