Fungi are microscopic organisms that don't need sunlight to survive. Some fungi have beneficial uses, while others cause illness and infection.
Nail fungal infections are typically caused by a fungus that belongs to a group of fungi called dermatophytes.
But yeasts and molds also can be responsible for nail fungal infections.
All of these microscopic organisms live in warm, moist environments, including swimming pools and showers. They can invade your skin through tiny invisible cuts or through a small separation between your nail and nail bed. They cause problems only if your nails are continually exposed to warmth and moisture — conditions perfect for the growth and spread of fungi.
Infection with nail fungus occurs more in toenails than in fingernails because toenails are often confined in a dark, warm, moist environment inside your shoes — where fungi can thrive.
Another reason may be the diminished blood circulation to the toes as compared with the fingers, which makes it harder for your body's immune system to detect and eliminate the infection.