Fungal infections of the foot are common. The bug that causes these infections love darkness and moisture, which is the exact environment that occurs inside the shoe. Even when a fungal infection is cured, the chances of it happening again are so high as the conditions that caused it in the first place are still present. The infection can be in the skin where is usually causes a spotty appearance that is itchy and is called tinea. When it affects the toenails, it is called onychomycosis and is very difficult to treat as the toenails get thick and protect the fungus from the topical drugs.