Nail fungus infections occur in the skin UNDERNEATH the nail, and not on the top surface of the nail itself. Because the nail is made of material that is generally water-tight, most medications used to treat skin fungus infections are incapable of going through the nail to kill the fungus. In order for nail fungus to be treated, the medication typically has to come to the infection from below the skin via the blood stream, or from above via an oil-based medication applied to the nail that will actually penetrate the nail tissue.