Hospital acquired pneumonia (HAP) is defined as new pulmonary infiltrate occurring after 1 week of hospitalization. Most patients have fever and leucocytosis but these may not necessarily be present. Hospitalized cases who develop pneumonia in less than five days and are called "early HAP" but are actually incubating community acquired pneumonia (CAP).

The pathogens which cause HAP are aerobic Gram negative bacilli. Pseudomonas aeruginosa is not the most common cause of HAP but is the most important organism in terms of mortality and morbidity.

The other common causes are Klebsiella, E.coli, S. pneumoniae and H. influenzae. Serratia, Acinetobacter and Legionella are the uncommon causes of HAP. Contrary to earlier belief, S. aureus is rarely, if at all, a cause of HAP. Other pathogens which are also very rare as a cause of HAP are Enterobacter, Stenotrophomonas and Burkholderia.

Inhalation, aspiration and hematogenous route are the three main modes of occurrence of HAP. Mechanical ventilation, intubation and reintubation increase the chances of HAP. Definitive diagnosis is only on lung tissue biopsy. Imaging studies may help in distinguishing from other conditions causing similar picture. Bronchoscopic techniques reflect more of airway colonization rather than lung pathology.

Empiric mono-antimicrobial therapy with cefepime or meropenem or piperacillin covers all common and uncommon organisms. For proven P. aeruginosa HAP, a double drug combination therapy is preferred using antimicrobial agents given as monotherapy along with either aminoglycoside (amikacin) or fluoroquinolone (levofloxacin) or monobactam (aztreonam). Total duration of treatment is for two weeks.