Fungi are microorganisms that exist with no sunlight. Some fungi are used for the people’s health improvement while others provoke infections.
A fungus of a dermatophyte group is responsible for fungal infections of nails. Molds and yeasts can also cause these infections. These microorganisms exist in damp and warm environments, like showers and swimming-pools. They can penetrate into your skin through microscopical cuts or separation between your nail-bed and nail. But they are dangerous only if your nails are always unprotected from dampness and warmth because these conditions are perfect for fungi to grow and breed.
Toenails are more often infected with fungus than fingernails, because we wear shoes and their environment inside is warm, dark and damp. One more reason is the lower circulation of the blood to the toes, which hinders your immune system from finding and eliminating the infection.