Although I'm not positive on the answere to your question, I would lean towards AAS not harming your immune system.
I've been in "the medical field" for 6 years and a hospital ER for 3. **All of this is my personal expirence, dont use me as a source, see your dr, ect**
Staph is a really strong bacteria, and has many forms that are resistant to antibiotics. Many people who get one infection get repeat infections. A large reason for that is because the bacteria wasnt fully killed.
Wound coultures arent always done. All antibiotics are not the same. Many people get secondary infections from surgery... I think you get where Im going with that.
Gyms, Public showers, and hospitals are all places where its easy to pick up this bug.
Basic Hygenie can help to. Staying clean, not picking at scratches, handwashing.
Also razors are known to transfer mrsa... do you shave your shins/legs?