Staphylococcus aureus is an opportunistic pathogen often carried asymptomatically on the human body. Methicillin-resistant S. aureus (MRSA) strains have acquired a gene that makes them resistant to nearly all beta-lactam antibiotics. Resistance to other antibiotics is also common, especially in hospital-associated MRSA. These organisms are serious nosocomial pathogens, and finding an effective treatment can be challenging. Community-associated MRSA strains, which originated outside hospitals, are also prevalent in some areas. While these organisms have generally been easier to treat, some have moved into hospitals and have become increasingly resistant to drugs other than beta-lactams. Animals sometimes become infected with MRSA from humans, and may either carry these organisms asymptomatically or develop opportunistic infections. Most of the MRSA found in dogs and cats seem to be lineages associated with people. Colonization of dogs and cats is often transient and tends to occur at low levels; however, these organisms can be transmitted back to people, and pets might contribute to maintaining MRSA within a household or facility. MRSA can also be an issue in settings such as veterinary hospitals, where carriage rates can be higher, especially during outbreaks in pets, horses and other animals.

Animal-adapted MRSA strains also exist. The livestock-associated lineage MRSA CC398, which apparently emerged in European pigs between 2003 and 2005, has spread widely and infected many species of animals, especially pigs and veal calves, in parts of Europe. CC398 has also been found on other continents, although the reported prevalence varies widely. People who work with colonized livestock or poultry can carry CC398, and these organisms can cause opportunistic infections. Other livestock associated MRSA have also been identified in various locations. CC9 is an especially prominent lineage in Asia.

MecC-bearing MRSA is a new type of MRSA first recognized in 2011. Many of these organisms have been recovered from animals, especially dairy cattle, but they can also infect and colonize humans. Recognizing mecC MRSA is currently problematic, as most of the diagnostic tests used routinely to identify MRSA do not detect these organisms.