Nails that are infected with a fungus may become discolored, thick and brittle, and may separate from the rest of the nail.

Fungal nail infection

Nails that are infected with a fungus may become discolored (yellowish-brown or opaque), thick and brittle, and may separate from the rest of the nail. In some cases, the nail may crumble. The dark, moist, and warm environment of shoes can promote fungal growth. In addition, an injury to the nail can put you at risk for a fungal infection. Fungal nail infections are difficult to treat. Medications applied directly to the nail are available, but they only help a small number of fungal nail problems. Oral medications (pills) may need to be prescribed by your doctor. Treatment also may include periodic removal of the damaged nail tissue.