A fungal infection of the nails (usually the same fungus that causes athlete's foot). Nails, usually the one on the big toe, becomes thick and misshapen with brittle, ragged edges. Sometimes it starts yellowing and dark spots appear where dead skin builds up underneath the nail.
The fungus thrives in humid, warm climates and also environments like swimming pools, locker rooms and showers. 10 to 15% of all adults have a fungal infection of the nails.
Nail fungus can be a sign of an overgrowth of the yeast called Candida albicans.