**3. Epidemiology**

Since methicillin‐resistant *S. aureus* constitutes a major burden on health care systems we will focus mainly on it. There are several terms for classifying MRSA infections, namely bacteremia. The first category is the health care–associated MRSA (also called nosocomial) that occurs more than 48 hours into hospitalization. The second category is community‐ onset health care–associated MRSA, which includes two factions: (1) patients in whom infection occurs less than 48 hours into hospitalization and (2) patients in the commu‐ nity who have had a prior hospitalization in the last 12 months (including for surgery or dialysis) or those who are residents of long‐term care facilities. The third category is community‐associated MRSA infections occurring outside of health care settings among individuals who do not have prior health care exposures. Several outbreaks of MRSA have occurred in the community without exposure to health care facilities. This reflects a great change in the epidemiology of MRSA‐related infections. Once solely a hospital pathogen and only seen among individuals with prior health care exposures, now MRSA is seen in populations without health care exposures. Poor hygiene conditions, close contact, con‐ taminated material, and damaged skin were found to be some of the risk factors for spread of MRSA infection in the community. In the United States, the most common MRSA com‐ munity‐acquired strain is the USA300 strain based on pulsed‐field gel electrophoresis. This community‐based clone mostly causes skin and soft tissue infections, but it may cause more invasive infections such as bacteremia in 5–10% of people. This clone is causing more nosocomial infections as well.

Besides being an important cause of community‐acquired bacteremia such as in cases of intravenous drug use leading to endocarditis, or cases of intravenous home infusion therapy, *S. aureus* is a leading cause of nosocomial bacteremia. It ranks second after coagulase negative staphylococci as a cause of primary bacteremia. In the hospital setting, a higher prevalence of methicillin‐resistant isolates is seen. Most of the time, bacteremia develops from *S. aureus* strains colonizing the host; however, this infection can be transmitted through contact with other colonized individuals or contaminated surfaces such as hands of health care workers or environmental spaces. Spread of staphylococci in aerosols of respiratory secretions from colonized patients has also been reported.
