MRSA can persist on most environmental surfaces. In hospitals and other health care facilities, it’s transmitted mainly by health care workers’ hands. However, outbreaks have also been reported in sports such as wrestling and rugby as well as in other types of close contact. Other populations at risk include military recruits, children in day-care facilities, prison inmates, homosexual men, and veterinarians who have contact with farm animals, especially pigs. The most common route of transmission in the community is thought to be through an open wound, such as a superficial abrasion, or from contact with a carrier. Other methods of transmission include poor handwashing, poor personal hygiene (not showering after workouts), sharing personal items (razors, towels, clothing), or failure to properly clean and disinfect exercise and training equipment.

Many colonized individuals become silent or asymptomatic carriers. The most frequent site of colonization is the anterior nares (40% of adults and most children become transient nasal carriers). Less common sites are the groin, axilla, and gut. Typically, MRSA colonization is diagnosed by isolating bacteria from nasal secretions.

In individuals in whom the natural defense system breaks down, such as after an invasive procedure, trauma, or chemotherapy, the normally benign bacteria can invade tissue, proliferate, and cause infection. Up to 90% of S. aureus isolates or strains are resistant to penicillin, and about 50% of all S. aureus isolates are resistant to methicillin as well as nafcillin and oxacillin. Strains may also have developed resistance to cephalosporins, aminoglycosides, erythromycin (E-mycin), tetracycline, and clindamycin (Cleocin).