HAP is usually caused by bacteria and it is currently the second most common nosocomial infection in the United States. It is associated with high mortality and morbidity (3). Available data suggest that HAP occurs at a rate of 5-10 cases per 1000 hospital admissions and the incidence increases by as much as 6- to 20-fold in mechanically ventilated patients (7-9). The exact incidence of VAP varies widely depending on the case definition of pneumonia and the population being evaluated (10). For example, the incidence of VAP may be up to two times higher in patients diagnosed by qualitative or semi-quantitative sputum cultures compared to quantitative cultures of lower respiratory tract secretions. It varies again between trauma patients and medical patients (7,11). HAP accounts for up to 25% of all intensive care unit (ICU) infections, and more than 50% of the antibiotics prescribed (12). VAP occurs in 9-27% of all intubated patients (7,13). In ICU patients, nearly 90% of episodes of HAP occur during mechanical ventilation. In mechanically ventilated patients, the incidence increases with the duration of ventilation. The risk of VAP is highest early in the course of hospital stay. It is estimated to be 3% per day during the first five days of ventilation, 2% per day during days 5 to 10 of ventilation, and 1% per day after this (14).