Fungal nail infections can be caused by three different types of fungus, alone or in combination.

• Dermatophytes are a type of fungus that can grow on the skin, hair, and nails. They do not invade the deeper tissues of the body. The most common dermatophyte, Trichophyton rubrum, causes most cases of athlete's foot infection, which in turn can infect the toenails. You can get infected by contact with objects that have dermatophytes on them, such as clothing, shoes, nail clippers, nail files, shower and locker room floors, and carpet. Dermatophytes cause about 90% of fungal toenail infections.¹
• Yeasts are a type of fungus that grows on the skin and nails. They are normally present on the human body. Various factors such as illness, antibiotic or birth control pill use, and immune system problems may allow an overgrowth of yeast, leading to a yeast infection.
• Molds (often called nondermatophytes) are a type of fungus that commonly grows in soil and can grow on the skin and nails. They are not usually passed between people.

Fungal infections are classified by where they begin and what they affect. Most fungal nail infections affect the skin under the nail (nail bed) and the nail itself (nail plate).

Toenails are more likely to become infected when ingrown or injured, as from frequent nail trimming. For more information, see the topic Ingrown Toenail.

A condition called onycholysis, the separation of the nail from the skin beneath, can increase the risk of fungal nail infections.

You can get a fungal nail infection when you come in contact with the fungus and it begins to grow on or under your nail. Fungi grow best in warm, moist areas, such as the area around the toes. But you can have fungi on your skin without developing a nail infection. You have to be likely to get the infection (susceptible) for it to develop. If you are susceptible to fungal infections, they tend to return, even after successful treatment and especially if you do not take preventive steps.

It is not clear why some people are more susceptible to fungal infections than other people.

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