Definition

The term pneumonia is rarely defined as a clinical entity in the literature, although it can be more clearly defined as a pathological term. However, it is generally agreed to be a lung infection with microorganisms in which the alveoli are involved, with resulting consolidation of the alveolar air spaces. Such a definition distinguishes pneumonia from bronchitis, which involves the airway mucosa and walls. Pneumonia results in impairment of the function of the affected lung tissue in its task of gas exchange, and the consolidation usually also causes shadowing of the affected lung tissue as seen on chest X-ray, so that some element of X-ray opacity is usually a requirement for the clinical diagnosis of pneumonia. In one study, the definition of X-ray opacity sufficient to define pneumonia required ‘either opacity at least involving a segment, or present in more than one lobe, which was neither pre-existing nor of other known cause’.

Some organisms can affect either the airways alone, causing bronchitis, or can involve primarily the alveoli causing pneumonia, but the distinction between bronchitis and pneumonia is not clear-cut, and in some infections there may be a combination of bronchitis and pneumonia in different degrees in different parts of the lungs in the same subject. Examples of such infections include those caused by organisms such as Streptococcus pneumoniae or Haemophilus influenzae.