Various types of fungi are present everywhere in the environment. The dark, moist surroundings created by shoes and stockings make the feet especially susceptible to fungal infection.
Most fungi are harmless until they penetrate the skin.
A fungus can invade through minor cuts, or after injury or repeated irritation to the toes have caused the nail to separate from the bed. Fungal infections of the nail plate and nail matrix are quite common. Fungus may cause the nail to thicken and become yellow or brownish. As the fungus grows, foul-smelling, moist debris can be seen. Pressure from a thickened nail or the build up of debris may make the toe painful.