Fungal nail infections (onychomycosis, or ringworm of the nail) occur when fungi invade a fingernail or toenail and the skin underneath the nail (nail bed). Toenails are more commonly affected than fingernails, and symptoms include cracked, yellow, discolored, streaked, thickened, or spotted nails.
Fungi that can cause this type of infection include dermatophytes, yeasts, and molds. Dermatophyte infections are spread by walking barefoot in public places (especially warm, damp areas such as swimming pools) and sharing personal items, such as towels and nail clippers. An athlete's foot infection also can spread to the nails, causing a fungal nail infection.
Fungal nail infections do not go away without treatment. They are treated with antifungal medicines, which are available in topical form (applied directly to the skin) or as pills. The pills are used for more severe infections. In rare cases, the nail can be removed. The condition tends to return, even after successful treatment.
WebMD Medical Reference from Healthwise