PDF Book Download - Overcoming Onychomycosis, by Scott Davis! Toenail fungus is a common fungal infection that can affect your toenails and can be difficult to treat. They usually like warm, moist, dark places, and infects one single nail by entering through a crack or opening. Nail fungus which is also known as onychomycosis infects your fingernails less frequently than they do the toenails. The condition usually isn’t painful, but it may make you feel self-conscious about how your foot looks.