Staphylococcus aureus (S aureus) is a type of bacterium. Like other kinds of bacteria, S aureus frequently lives on the skin and in the nose without causing health problems. Staphylococcus aureus becomes a problem when it is a source of infection in the skin, lungs, or blood. These bacteria can be spread from one person to another through casual contact or through sharing contaminated objects. Methicillin-resistantStaphylococcus aureus (MRSA) bacteria are resistant to commonly used antibiotics, the medicines used to treat bacterial infections. Because of this, MRSA infections are more difficult to treat than ordinary S aureus infections. MRSA that is acquired in a hospital is called hospital-associated MRSA (HA-MRSA). MRSA infections are now becoming more common in healthy, nonhospitalized persons. These infections can occur among young people with cuts or wounds who have close contact with each other, such as members of sports teams. This type of MRSA is called community-acquired MRSA (CA-MRSA). This Patient Page is based on one originally published in the October 17, 2007, issue of JAMA.