Mycosis is any disease caused by a fungus. Onychomycosis is infection of the toenail with fungus.

Skin fungi not only can disfigure the nails but can also cause physical pain and impair the ability to work. Nail fungi affect toenails four times more often than fingernails. Once fungi take hold, it may spread from nail to nail, foot to foot, and rarely to the hands and fingernails.

When the nails are exposed to a warm moist environment, a fungus can develop on the nail or under its outer edge....