Staphylococcus aureus (S. aureus), often referred to as "staph," is a bacterium commonly carried on the skin or in the nose of healthy people. S. aureus typically causes a skin infection, but can cause infections in the bloodstream and major organs. Methicillin-resistant S. aureus (MRSA) occurs when the bacteria become resistant to the antibiotic, methicillin and other more common antibiotics such as oxacillin, penicillin and amoxicillin, making it more difficult to treat. The rate of invasive MRSA (infections in typically sterile sites like the bloodstream) is estimated to be 32 per 100,000 persons in the United States; the mortality (death) rate is thought to be about 6 per 100,000 persons. The risk of invasive MRSA infections is highest among older individuals, Blacks/African Americans, and men.

MRSA infections were initially limited to hospitals and nursing homes, especially among patients with weakened immune systems. Since the 1980s, community-acquired cases and outbreaks also have been reported. Community acquired cases are those not related to past year hospitalization or medical procedures like dialysis, surgery, or catheterization. These infections typically occur among otherwise healthy individuals and are more likely to be limited to skin infections. An increase in the virulence of MRSA bacteria in the past decade, however, has been responsible for more severe and sometimes fatal community acquired infections. More recently, MRSA has been identified in food animals and a few outbreaks have been ‘food-initiated’ or foodborne. In one such outbreak, those affected developed typical foodborne illness symptoms, such as vomiting and stomach cramps.