Ventilator-associate pneumonia (VAP) is a sub-type of hospital-acquired pneumonia (HAP) which occurs in people who are on mechanical ventilation through an endotracheal or tracheostomy tube for at least 48 hours. VAP is a medical condition that results from infection which floods the small, air-filled sacs (alveoli) in the lung responsible for absorbing oxygen from the atmosphere. VAP is distinguished from other kinds of infectious pneumonia because of the different types of microorganisms responsible, antibiotics used to treat, methods of diagnosis, ultimate prognosis, and effective preventive measures. In the community pneumonia is most often caused by S. pneumoniae, H. influenzae, or S. aureus. However, in the hospital the organism associated with pneumonia is most often Pseudomonas, regardless of whether or not the patient is ventilated. In order for a patient to have VAP they have to be on a ventilator; it’s not the organism that distinguishes VAP, it's the fact that the patient is on a ventilator.