Staphylococcus aureus, often referred to simply as "staph," is a bacteria commonly carried on the skin or in the nose of healthy people. Approximately 25% to 30% of the population is colonized (when bacteria are present, but not causing an infection) in the nose with staph bacteria. Sometimes, staph can cause an infection. Staph bacteria are one of the most common causes of skin infections in the United States. Most of these skin infections are minor (such as pimples and boils) and can be effectively treated by drainage of pus with or without antibiotics (also known as antimicrobials or antibacterials). However, staph bacteria also can cause serious infections (such as surgical wound infections, bloodstream infections, and pneumonia).

While 25% to 30% of the population is colonized with staph, approximately 1% is colonized with MRSA.

Colonization = organism is on or in the body but not causing disease.
Infection = organism is present and causing signs and symptoms of disease.

Most MRSA infections are skin infections that may appear as pustules or boils that often are red, swollen, painful, or have pus or other drainage. These skin infections commonly occur at sites of visible skin trauma, such as cuts and abrasions, and areas of the body covered by hair (e.g., back of neck, groin, buttock, armpit, beard area of men).

Almost all MRSA skin infections can be effectively treated by drainage of pus with or without antibiotics (also known as antimicrobials or antibiotics). More serious infections, such as pneumonia, bloodstream infections, or bone infections, are very rare in healthy people who get MRSA skin infections.