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 used in combination with topical medication works best to treat fungal nail infections. Sometimes, office-based procedures are utilized to remove infected areas of the nail.