Many healthy people carry Staphylococcus aureus bacteria on their skin. These carriers are said to be colonized: they are not infected, but the bacteria are present on their skin or inside of their nose. Staph bacteria are generally harmless unless they enter the body through a cut or other wound. Even then, they typically cause only minor skin infections in healthy people. But sometimes, usually in older adults and people who are ill or have weakened immune systems, ordinary staph infections can cause serious illness. Some otherwise healthy people also contract serious infection, a trend that has been on the increase in recent years.

MRSA and other staph infections are spread through physical contact with someone who is infected with MRSA or who is a carrier and through contact with objects such as door handles, floors, sinks, or towels that have been touched by a MRSA-infected person or carrier. In some cases staph bacteria can enter healthy intact skin, but more often, the bacteria gain entry through cuts, abrasions, or other skin flaws such as psoriasis. MRSA can also affect tissues inside the body and while this sometimes occurs as a complication of a MRSA skin infection, in many cases such infections are introduced by surgical instruments, implanted medical equipment, or large lacerations.

Until recently, almost all cases of MRSA infection occurred inside hospitals. However, MRSA infections contracted outside of hospitals, called community-associated MRSA (CA-MRSA) are becoming more common. Such severe infections must be treated by administering intravenous antibiotics and may require surgery to remove portions of tissue infected with MRSA bacteria.

As with all antibiotic resistant strains of bacteria, part of the blame for the existence of MRSA lies with decades of excessive and unnecessary antibiotic use. The prescribing of antibiotics for conditions that are not confirmed to be caused by bacteria and the improper use of antibiotics by patients (such as not taking entire the recommended course because symptoms have improved) both make antibiotic resistant strains of bacteria more likely to arise and thrive. The use of antibiotics in foods may also contribute to this problem. But even if antibiotics are used judiciously, the tendency of bacteria to reproduce quickly and mutate often makes resistance to them an eventuality.