HAP, VAP, and HCAP can be caused by a wide spectrum of bacterial pathogens, may be polymicrobial, and are rarely due to viral or fungal pathogens in immunocompetent hosts (7,8,15,58-62). Common pathogens include aerobic Gram-negative bacilli such as Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae or Acinetobacter species. Infections due to Gram-positive cocci such as Staphylococcus aureus, particularly Methicillin-resistant Staphylococcus aureus (MRSA), have been rapidly emerging in the United States over the past decade (12,63). Pneumonia due to Staphylococcus aureus is more common in patients with diabetes mellitus, head trauma, and those hospitalized in ICUs (64). This table provides the most common causes of HAP, VAP, and HCAP according to the onset of infection and risk factors for MDR(45).