An infection of the lungs where the air sacs (alveoli) fill with fluid and pus making breathing difficult. Bacterial pneumonia can follow from an infection by respiratory viruses such as influenza.

Community-acquired pneumonia (CAP) affects people outside hospital. Pneumonia can also occur in patients while in hospital (hospital-acquired pneumonia, HAP), particularly when patients are on mechanical ventilation (ventilator-associated pneumonia, VAP).

Pneumonia can be caused by bacteria, viruses and fungi. The organisms causing bacterial community-acquired pneumonia are usually different to those responsible for hospital-acquired pneumonia.