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INTRODUCTION

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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”

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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.

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✺ Viral infections account for most of acute morbidity associated with respiratory diseases

✺ Respiratory viruses are represented from different virus families

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The viruses that are major causes of acute respiratory disease (ARD) include influenza viruses, parainfluenza viruses, respiratory syncytial virus (RSV), adenoviruses, rhinoviruses, respiratory coronaviruses, human metapneumovirus (hMPV), and bocaviruses (a member of parvovirus group). Reoviruses can also affect respiratory tract and are included in this chapter. Other viruses, such as enterovirus, measles virus, Epstein-Barr virus (EBV), cytomegalovirus (CMV), varicella-zoster (VZV), herpes simplex virus (HSV), and hantavirus can also cause respiratory symptoms but are discussed in other chapters of their principal diseases.

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✺ Major acute respiratory disease causing viruses include influenza virus, parainfluenza virus, respiratory syncytial virus, human metapneumovirus, coronavirus, and rhinovirus

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In addition to the ability to cause a variety of ARD syndromes, this group of viruses discussed in this chapter 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. These respiratory viral agents are associated with an increased risk of bacterial superinfection of the damaged tissue of the respiratory tract, and all have a worldwide distribution.

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✺ Transmission of respiratory viruses by droplet nuclei or hand transfer of contaminated secretions

✺ Short incubation period of approximately 1 to 4 days

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INFLUENZA VIRUSES

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INFLUENZA VIRUSES Overview

Three types of influenza viruses (A, B, and C) infect humans. Influenza virus types A and B both cause more severe symptoms than does influenza virus type C. Influenza virus A, which has several subtypes based on hemagglutinin (H) and neuraminidase (N), undergo more genetic changes than types B and C. Influenza viruses are enveloped, negative-sense segmented RNA virus that replicate in the nucleus of the infected cells. Direct droplet spread is the most common mode of transmission and the incubation is period is about 2 days. The virus multiply in ciliated respiratory epithelial cells, ...

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