Nail fungus, or onychomycosis, is an infection that occurs under the toenail, or less commonly the fingernail. It can start when your nail comes in contact with one of the varieties of fungi capable of infecting the skin and nails. The fungus that causes athlete’s foot is also a common cause of nail fungus. However, there are several different types of fungal organisms responsible for nail fungus, including molds and yeasts. The most common organism that can cause nail fungus is called Trichophyton rubrum, which is a dermatophyte.
In the early stages of an infection, nail fungus typically appears as a yellow or white spot underneath the tip of the nail. If the infection continues to spread, it can cause the nail to become thick and yellow or dark in color. The nail may become distorted and crumble, break or crack. In some cases, people with nail fungus notice a foul odor.