What causes athlete's foot?

My guy keeps his feet very clean, but he wears leather work boots on a daily basis. He hasn't had athletes foot for years until he recently purchased these boots. They were not tried on by someone else prior to his purchase. He has been no where near a place where he could get it from someone or a contaminated surface. How could he have gotten it?

His socks are new.

Athlete's foot is a fungus known as tinea pedis. When the fungus infects the upper body, this is known as tinea corporis, or ringworm, causing raised and reddened rashes with clear centers. Athlete's foot is commonly found in athletetic showering areas and long-term athlete's foot often involves the toenails and can totally destroy the nail plate and/or enter the bone, Blistering can occur when the immune system becomes sensitized to the fungus. It can usually be found between the toes where the skin has a greater tendency to crack, blister, or become sore and itchy. The infection will persist indefinitely until treatment is effective. The general health of the person tends to determine how serious the infection and extent of the symptoms.
Products with Tea Tree oil are highly effective in the treatment of foot and toenail fungal infections. It is much more effective than griseofulvin, which has been commonly used. If the two are used together, the griseofulvin may lessen in effectiveness. It is also important to note that griseofulvin may be toxic to the liver and liver enzyme functions must be monitored during therapy with this drug. Several years ago in a report issued by The Physician and Sportsmedicine, it was stated that in some cases irritation was misdiagnosed as athlete's foot when it is really an allergic reaction to a chemical in sneaker insoles. This "contact dermatitis" can be differentiated from athlete's foot because the reaction occurs mainly on the ball, outersides and top of the foot where it comes in contact with the shoe Athlete's foot usually appears in the cracks between the toes.
If you have contact dermatitis, an entirely different method of treatment is required. With contact dermatitis, you must NOT use any pure Tea Tree oil.
To eliminate Athlete's foot, clean the affected area well with antibacterial soap and water. Be sure to wear clean white cotton (if possible) socks daily. these should be washed only using products that do not contain chemical fillers (grocery and discount store national brands) because they often contain harmful chemicals. Each time you change your socks, dribble a few drops of Tea Tree oil on the clean socks. Rub them together before putting them on. To avoid re-infection, always wear bathing slippers in public swimming pools, saunas, and showers as well as in dorms and hotels. Dry your feet carefully after washing. Avoid wearing polyester socks and shoes not made of leather. Do not wear rubber boots or tennis shoes the entire day and change your socks every day. To help avoid infecting other family members spray shower and bathtub surfaces as well as bathroom floors with a good chemical free disinfectant.

got to keep those feet dry. try foot powder

humidity in between the toes causes this treatable fungal infection, good luck

It could be a recurrence of the old infestation. I not sure but I think the fungus can survive for quite some time in a dormant stage. Otherwise he has contacted the spores somewhere.

He might try alternating the work boots along with the usual treatment with an anti fungal. Perhaps letting them completely dry out between wearings will help.

Fungus from under the toenails.. keep nails short and dry feet well. Also wear sandals at campgrounds.
Peace