"What are fungus toenails? Fungus toenails are caused by a fungal infection that lives in and under your nails. Fungal toenails are not caused by poor hygiene. Fungus likes to live in warm, moist places like your shoes. You can also get a fungal infection from trauma, a puddle at the local pool, or from a nail salon. Toenail fungus can also be passed from person to person, so it is very important that you have this condition treated as soon as possible."