Nosocomial pneumonia is an important hospital-acquired infection that occurs 48 hours or more after hospital admission. It is the leading cause of death from nosocomial infections. A subset of nosocomial pneumonia occurs in patients receiving mechanical ventilation. These patients have what is termed ventilator-associated pneumonia (VAP). The use of a ventilator by the patient increases the risk of developing pneumonia by a factor of 10. As a result, VAP frequently occurs in patients requiring significant mechanical ventilatory support such as patients with acute respiratory distress syndrome (ARDS), shock syndromes, and multiorgan failure.

It is important to understand that the etiology of VAP is dependent upon the fundamental relationship between the host, the microbial agent, and the environment. The host refers to the intensive care unit (ICU) patient who may be at particular risk of acquiring a nosocomial infection. Alterations in underlying host defenses like the endotracheal tube, which suppresses the normal ability of the body to eliminate respiratory pathogens by coughing; host diseases like cancers; and underlying respiratory diseases such as cystic fibrosis affect both the colonization of and predisposition to VAP. The microbial agents themselves have specific properties that increase their pathogenicity. Pili, microbial toxins, and adherence factors all play a role in the ability of particular organisms to have a propensity to cause diseases. Gram-negative bacteria are more likely than gram-positive organisms to cause VAP (60% vs. 40%). The gram-negative microbial organisms that are responsible for causing VAP include Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae, and Acinetobacter species. The gram-positive spectrum primarily involves Staphylococcus species. In addition, the environment of the ICU, the hospital, and the region will all affect the spectrum of microbials that colonize the environment and predispose the patient to infections.