MRSA infection is caused by Staphylococcus aureus bacteria — often called "staph." MRSA stands for methicillin-resistant Staphylococcus aureus. It's a strain of staph that's resistant to the broad-spectrum antibiotics commonly used to treat it. MRSA can be fatal.
Most MRSA infections occur in hospitals or other health care settings, such as nursing homes and dialysis centers. It's known as health care-associated MRSA, or HA-MRSA. Older adults and people with weakened immune systems are at most risk of HA-MRSA. More recently, another type of MRSA has occurred among otherwise healthy people in the wider community. This form, community-associated MRSA, or CA-MRSA, is responsible for serious skin and soft tissue infections and for a serious form of pneumonia.
MRSA infection - MayoClinic.com