Staphylococcus aureus is an important bacterial cause of contagious mastitis on dairy farms worldwide. More importantly, it is often at the root of chronically high somatic cell counts, recurrent clinical mastitis, and damaged mammary gland tissue. It is considered to be a contagious udder pathogen that spreads within and between cows during milking. Because it is often subclinical (milk looks normal but with a potentially high somatic cell count), infected animals pose a risk of infection to herd mates during each milking.