Hospital acquired pneumonia.

Hospital acquired pneumonia (HAP) is defined as pneumonia that occurs 48 hours or longer after hospital admission and excludes any infection that is incubating at the time of admission1. It is also commonly termed nosocomial pneumonia. Ventilator-associated pneumonia is widely recognised as pneumonia developing after at least 48 hours of mechanical ventilation (MV), and can be considered a subgroup of HAP with distinct differences in terms of pathogenesis, histology, aetiology and prognosis. The concept of ‘early-’ and ‘late-onset’ HAP is also useful: • Early-onset HAP is commonly defined as occurring within four days of hospitalisation, with Streptococcus pneumoniae, Haemophilus influenzae and Staphylococcus aureus the most frequently isolated organisms2,3. • Late-onset HAP, occurring five or more days after hospitalisation, is caused by pathogens such as enteric Gram-negative bacilli that have replaced the ‘community’ pathogens in the oropharynx. Patients readmitted to hospital with pneumonia following recent hospital discharge may have features more consistent with HAP than with community acquired pneumonia (CAP).