The type of fungus that causes these infections typically belongs to a group called dematophytes. This fungus thrives in dark, moist environments like locker rooms and public swimming pools. In many cases, your toenails and feet are the perfect breeding ground for these fungi due to their confinement in shoes for extended periods of time. Your shoes are dark, warm, and can also be wet from sweat- the three factors that bacteria and fungus love!
If you wear the same shoes every day, you are at an increased risk for fungal infections. By wearing the same pair, you are not allowing time for the inside of the shoes to air out. Fungus is much less likely to thrive in an environment that is dry and cool. Your fingernails, however, are not usually confined to dark, warm places for long periods of time, and are washed regularly.