These are divided into two anatomically separate categories. First, there are those of the upper respiratory tract, which extends from the nose to the vocal cords. These include the common cold (coryza), sinusitis, pharyngitis, laryngitis and epiglottitis. Secondly, there are infections of the lower respiratory tract. These may affect the large airways (bronchitis), the alveoli and parenchyma (pneumonia) or the pleura space, leading to an empyema. The source of infection is variable. Droplet inhalation is the most frequent although pathogens may be introduced to the lung by alternative routes, such as the aspiration of pharyngeal contents as seen in neurological conditions leading to bulbar palsy and defective swallowing, hematogenous spread as in miliary tuberculosis or staphylococcal septicemia, and direct extension from surrounding tissues. In patients with comorbid illness, impaired host responses or damaged respiratory tract, organisms normally regarded as nonvirulent can become pathogenic and cause
Table 1 Community-acquired pneumonia in British hospitals
Proportion of microbiological diagnoses (% total)
Streptococcus pneumoniae 34
Mycoplasma pneumoniae 18
Influenza A virus 7
Haemophilus influenzae 6
Others Legionella pneumophila Streptococcus species (others) Coxiella burnettii Staphylococcus aureus Enterobacteriaceae Microbiologically negative 33
Adapted from the British Thoracic Society/Public Health Laboratory Sen/ice (1987).
infection. This is common in some immunocompromised conditions such as the acquired immune deficiency syndrome (AIDS).
The commonest cause of pneumonia encountered in the community is the bacterium Streptococcus pneumoniae, followed by Mycoplasma pneumoniae and influenza virus (see Table 1). The diagnosis is made clinically and with the aid of investigations such as chest radiographs (Figure 1), an elevated white cell count and the growth of an organism on culture of sputum, respiratory secretions (Figure 2) or blood. Indirect techniques such as complement fixation or indirect immunofluorescence tests are
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