Colonization occurs when a patient has MRSA in or on a body site but has no clinical signs or symptoms of disease. A person colonized with MRSA may be a temporary or a longer term carrier of MRSA. Certain carriers may be shedders of MRSA [e.g., patients with dermatitis or burns].

Infection occurs when MRSA enters a body site and multiplies in tissue causing clinical manifestations of disease. This is usually evident by fever, a rise in the white blood cell count, or purulent drainage from a wound or body cavity. The distinction between colonization and infection is a clinical one. Such a distinction should be determined by the clinician, not by culture results alone.

Colonized and infected patients are the major reservoirs of MRSA. Colonization often occurs in the nares (nose), axillae (arm pits), chronic wounds, perineum or around gastrostomy and/or tracheostomy sites. Patients at risk for MRSA colonization are generally debilitated patients who may have prolonged hospitalizations, chronic wounds, or received treatment with multiple antibiotics.