Presuming it is actually a
fungus, there is a problem because it is under the
nail. Outside, on the skin for instance, you would just use Vick's-Vapo-Rub (believe it or not). Getting under the nail is a problem. There are systemic anti-fungals, but my husband did not have much luck with that. He kept painting the nails and pushing stuff under (as far as he could). I think it is pretty well gone now, but it took a long, long time.