Black toenail fungus is called onychomycosis. The black coloring of the fungus is caused by debris that builds up underneath the nail. This type of nail fungus is typically caused by dermatophyte fungus, although other causes include yeast or mold. The signs and symptoms of onychomycosis include nails that are thickened, distorted in shape, and brittle or ragged. Nails that are infected often separate from the nail bed.
Toenail fungus enters the skin through tiny cuts or separations between the toenail and the nail bed. The condition is typically worse if nails are often exposed to warm, moist conditions in pools and showers. Toenails are more susceptible to fungal infections because there is less blood flowing to the toes. This makes it harder for the immune system to detect and prevent the infection. Once toenails become infected, the fungus that causes the condition thrives inside of shoes because they create an environment that is often warm and moist. The risk of contracting toenail fungus is increased by advanced age, heavy perspiration and having a family history of nail fungal infections. Men are also more susceptible to nail fungal infections as are people who live with someone who has nail fungus.