Staphylococcus aureus is a type of bacteria that lives on healthy people's nose and skin. It does not usually cause problems. However, if there is a break in the skin or if a person is sick, the bacteria can infect the skin or the tissues underneath it. In severe cases, it can enter the bloodstream and cause infection elsewhere in the body. Doctors have used antibiotics to treat regular S. aureus for so long that some kinds of the bacteria have developed resistance. Infections with antibiotic-resistant S. aureus are also called methicillin-resistant S. aureus (MRSA) infections. Once, MRSA infections were found only in patients who had spent time in a hospital. But now MRSA infections are common in otherwise healthy people who have never been treated with antibiotics and who have never spent time in a hospital (community-acquired MRSA). In addition, in the past 2 years, doctors in San Francisco and Boston have found forms of MRSA that are resistant to multiple antibiotics (multidrug-resistant MRSA). Multidrug-resistant MRSA infections were more frequent in gay men and in people with HIV infection. If MRSA is becoming less responsive to antibiotics and is spreading into the community, otherwise healthy people may develop infections that are difficult to treat.