Fungal nail infection
Thick, brittle, yellow-brown, or opaque nails are common with fungal nail infections. The infected area may crumble or seem to pull away from the rest of the nail. Fungus thrives in the warm, moist, dark environment created by wearing closed-toed shoes. Nail injury also increases the risk of fungal nail infection. These infections are difficult, but not impossible, to treat. Oral medications work best to treat fungal nail infections. Topical treatments are only effective for a few types of fungal nail infections. Sometimes, surgery is necessary to remove infected areas of the nail.