Toenail fungus, also called onychomycosis, is a relatively common condition that disfigures and sometimes destroys the nail. This problem can be caused by several different types of fungi (microscopic organisms related to mold and mildew). These fungi thrive in the dark, moist and stuffy environment inside shoes. As they grow, fungi feed on keratin, the tough protein that makes up the hard surface of the toenails. In most cases, the fungus belongs to a group of fungi called dermatophytes, which include Trichophyton rubrum and Trichophyton interdigitale. Other, less common causes of onychomycosis include yeasts and molds. Toenail fungus affects 2% to 18% of all people worldwide and 3% to 5% of people in the United States. It is relatively rare in children, affecting only about 1 out of every 200 people younger than 18. However, the likelihood of getting toenail fungus increases with age. Up to 48% of people have at least one affected toe by the time they reach age 70. Although 2.5 million Americans see a podiatrist annually for treatment of toenail fungus, many more are infected but never seek help. Some people consider toenail fungus just a cosmetic problem and don't bother seeking treatment. Almost anyone who wears tight-fitting shoes or tight hosiery is more likely to develop toenail fungus, especially if they also practice poor foot hygiene. Another risk is wearing layers of toenail polish, which doesn't allow the nail to breathe. Also, because toenail fungi may spread from foot to foot on the floors of showers and locker rooms, fungal infections of the toenails are especially common among military personnel, athletes and miners. The condition also tends to affect people with chronic illnesses, such as diabetes or HIV, as well as people with circulatory problems that decrease blood flow to the toes. However, many people have no clear risk factors. Of all the toenails, those on the big toe and little toe are the most likely to develop a toenail fungus. This may be partly because the big toe and little toe are constantly exposed to the mild trauma of friction from the sides of shoes. Prevention Wear comfortable shoes and hosiery that allow your feet some "breathing space." Wear shoes, sandals or flip-flops in community showers or locker rooms. Wash your feet every day, dry them thoroughly, and use a good-quality foot powder. Wear clean socks or stockings every day. Keep your toenails trimmed. Disinfect pedicure tools before you use them. http://www.intelihealth.com/IH/ihtIH/WSIHW000/9339/24470.html

you kinda answered your own question. fungus causes fungus. :P