Respiratory infections are the most common afflictions of humans, and most are caused by viruses. Children contract on average about half a dozen respiratory illnesses each year, and adults perhaps two or three. Admittedly these are mainly trivial colds and sore throats, but they account for millions of lost working hours and a significant proportion of all visits to family physicians. More serious lower respiratory tract infections tend to occur at the extremes of life, and in those with preexisting pulmonary conditions. The most important human respiratory viruses are influenza and respiratory syncytial viruses (RSV), the former killing mainly the aged and the latter the very young. Of the estimated 5 million deaths from respiratory infections in children annually worldwide, at least 1 million are viral in origin.
Altogether, there are about 200 human respiratory viruses, falling mainly within six families: orthomyxoviruses, paramyxoviruses, picornaviruses, co-ronaviruses, adenoviruses, and herpesviruses. Here we shall confine ourselves to those that enter the body via the respiratory route and cause disease confined largely to the respiratory tract. Many other "respiratory" viruses are disseminated via the bloodstream to produce a more generalized disease, as is the case with most of the human childhood exanthems such as measles, rubella, and varicella. Other viruses, entering by nonrespiratory routes, can reach the lungs via systemic spread, and pneumonia may represent the final lethal event, as in overwhelming infections with herpesviruses or adenoviruses in immunocompromised neonates or AIDS patients.
Systemic viral infections such as measles generate a strong memory response and prolonged production of IgG antibodies, which protect against reinfections for life. In contrast, viruses that cause infection localized to the respiratory tract with little or no viremia, such as RSV or rhinoviruses, induce only a relatively transient mucosal IgA antibody response; hence, reinfections with the same or a somewhat different strain can recur repeatedly throughout life, in addition, numerous strains of viruses like influenza virus arising by antigenic drift may cause sequential episodes of the same disease in a single patient.
Whereas some viruses have a predilection for one particular part of the respiratory tract, most are capable of causing disease at any level, and the syndromes to be described below overlap somewhat (Fig. 36-1). Nevertheless, for ease of description we will designate six basic diseases of increasing severity as we descend the respiratory tract: rhinitis, pharyngitis, croup, bronchitis, bronchiolitis, and pneumonia (Table 36-1).
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