Boils are usually caused by Staph Aureus. Hopefully you dont have MRSA - methicillin-resistant S.aureas, which is quite common these days and can be more persistent. At the county hospital setting, bad skin infections are assumed to be MRSA. When I was in med school MRSA was thought to be in hospitals and nursing homes only. If it doesnt go away with the above mentioned treatments, you may want someone to look at it.