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What Causes Foot & Toenail Fungus?

By Martin Hughes ; Updated July 18, 2017

There are several principal causes of foot and toenail fungus. According to the U.S. National Library of Medicine and the National Institutes of Health, fungal toenail infections are common in adults and typically follow fungal infection of the feet. The NIH notes that fungal foot and nail infections can be difficult to treat, and that even with successful treatment, the fungus may still return.

Conventional Footwear

Conventional footwear can encourage the growth and proliferation of foot and toenail fungus. According to Dr. Ray McClanahan, a sports podiatrist in Portland, Oregon, foot and toenail fungus, including athletes foot, is commonly seen in people who wear constrictive shoes or boots that jam the toes together and allow the foot to become hot and sweaty. McClanahan notes that an athlete may be particularly susceptible to fungal infections, due to the accumulated sweat on his feet and in his shoes.

Foot and toenail fungus thrives in the warm, moist and dark environments of conventional footwear, but sandals and other open-toed shoes expose foot and toenail fungus to lethal ultraviolet rays. If a person is living in an environment that precludes the wearing of sandals, he should consider keeping several sets of dry socks on hand and periodically changing his socks when the first signs of moisture arise. Some newer sock models are made of synthetic material that help wick moisture away from the skin, inhibiting fungal skin invasions.

Decreased Immune Function and Circulation

Foot and toenail fungus are more likely to occur in people who have decreased immune function or lower extremity circulation problems, such as those with HIV/AIDS and diabetes, respectively. According to the Mayo Clinic website, having diabetes, circulation problems or a weakened immune system significantly increases a person's risk for foot and toenail fungus. The Mayo Clinic website notes that HIV infection weakens the immune system, making a person susceptible to bacterial, viral, parasitic and fungal infections.

Diabetes progression causes diminished blood flow to the feet and toes, which creates an optimal situation for fungus to thrive, as the body's immune system has a more difficult time detecting and eliminating the infection. Both circulation and immune functions may diminish with age, and nail fungus is more common in older adults as a result. The Mayo Clinic website reports that along with diminished circulation and immune function, more years of fungi exposure may help explain its greater prevalence in older adults.


Toenail trauma may contribute to toenail fungus. According to, fungal infections of the toenail or onychomycosis occurs when microscopic fungi invade through a cut or break in the nail due to trauma. notes that athletes and people who wear tight-fitting shoes or tight hosiery can suffer toenail trauma and are at a higher risk for onychomycosis. Once a fungal infection occurs in the toes, it may cause further problems, such as ingrown toenails, according to the American College of Foot and Ankle Surgeons.

A 2006 study by Jason A. Winston and Jami L. Miller, M.D. published in the journal "Clinical Diabetes" concludes that repeated trauma to the nails increases the risk of onychomycosis. Winston and Miller also note that diabetic patients suffering from decreased foot sensation are more likely to experience toenail trauma, which damages the nail and opens portals of entry for toenail fungus.

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