Thick, yellow nails are usually caused by the presence of a fungal nail infection.    A fungal infection of the toe nails is a common foot problem.   Nail fungus (onychomycosis) is an infection of the nail plate and nail bed.   It is caused by various types of fungi, which are commonly found throughout the environment. Fungi are simple parasitic plant organisms that do not require sunlight for growth.   A group of fungi called dermatophytes have evolved to attack human nails and skin. The infection of the nail begins with some type of trauma to the nail. The dermatophyte gains entry into the nail and begins to feed on the nail protein called keratin. The big toe nail is the most commonly infected toe nail.   Our big toe nails can get damaged or traumatized from:   The way we walk or run. ( because of a unstable low arched foot or bunion) The   shoes we wear ( tight fitting shoes or high heels)   The type of job (the requireme...