Methicillin-resistant Staphylococcus aureus: Staphylococcus aureus (a type of staph bacteria) resistant to the antibiotic methicillin. Abbreviated MRSA. MRSA first appeared among people in hospitals and other health facilities, especially among the elderly, the very sick, and those with an open wound, an IV catheter, or a urinary catheter. MRSA has since been found to cause illness in the community outside of hospitals and other health facilities. Community-associated MRSA infections typically cause skin lesions (such as boils), but can also cause severe illness. The transmission of MRSA is largely from people with active MRSA skin infections. MRSA is almost always spread by direct physical contact, and not through the air. Just as S. aureus can be carried on the skin or in the nose without causing any disease, MRSA can be carried in this way as well, a process known as colonization. MRSA infections are usually mild superficial infections of the skin that can be treated successfully with proper skin care and antibiotics. However, MRSA can be difficult to treat and can progress to life-threatening blood or bone infections because there are fewer effective antibiotics available for treatment.