The World Health Organization (WHO) report that 1.4 million people globally each year die as a result of pneumonia.¹ Mortality is high among the most susceptible: the elderly, infants, immunocompromised patients and those taking immunosuppressive drugs, smokers and patients with obstructive lung diseases. Patients admitted to acute care are placed at increased risk, as pneumonia may also occur as a complication in hospital settings secondary to a procedure such as surgery or artificial ventilation.

Acute pneumonia is a respiratory infection which results in inflammation of the interstitial lung tissue. It may be the primary result of an infection or secondary to another disease, e.g. malignancy or AIDS, or it may be due to aspiration of vomit or mucus. The condition is generally classified into two distinct groups classified according to where the infection was acquired:

• Community-acquired pneumonia (CAP).
  
• Hospital-acquired pneumonia (HAP) (nosocomial).

Causative agents may be bacterial or viral, and their symptoms are similar (Figure 18.1), but may be more numerous in viral presentations. Untreated pneumonia may cause rapid deterioration, requiring urgent care escalation (see red flag box) and possibly intubation for mechanical ventilation in intensive care.

Community-acquired pneumonia is defined as an infection that is not acquired in hospital, or after recent hospitalisation. It is commonly caused by the pneumococcal bacterium Staphylococcus pneumonia, Staphylococcus aureusi, or other gram-negative bacteria such as Klebsiella, Pseudomonas, or E. coli. Fungal causes include Pneumocystis jurovecii pneumonia. CAP can further be divided into ‘typical’ and ‘atypical’ presentations (Table 18.1).