Nails that are infected with a fungus may become discoloured (yellowish-brown or opaque), thick, and brittle and may separate from the rest of your nail. In some cases, your nail may crumble. The dark, moist, and warm environment of shoes can help a fungus grow. An injury to your nail can also lead to a fungal infection. Fungal nail infections are hard to treat. Medications applied to the nail are available, but they only help a small number of fungal nail problems. You may need prescription medicine that you take by mouth. Your doctor might also remove the damaged nail.