Staph aureus is one of the most bacteria around. We all carry it around with us on our skin. It usually only causes a problem if broken skin or cuts (for eg) aren't cleaned.
Any time you get a cut which is "infected" more than likely it will be Staph which is doing the infecting.
You have probaby heard of the methicillin resistant Staph aureus infection, which is usually shortened to MRSA. this is the version of the bug whcih causes problems in hospitals as it is resistant to a most antibiotics, including the hight potency methicillin (I've never personally known a horse get MRSA though )
yes, it is a secondary bacterial skin infection. Secondary to the dermatophiles bacterial which is found in rain scald or sweat scald (when on the back area) or mud fever when on the legs. The dermatophiles needs to be addressed appropriately to prevent staph developing. If it is on the back it may be due to over rugging as we have had a mild, wet winter. It thrives in a warm moist environment.