Toenail fungus is highly contagious and eventually all the toenails will likely become infected. As the infection spreads to neighboring toenails it is increasingly difficult to treat the problem. Eventually that single infected toenail will likely turn into ten thickened discolored (dark or yellow) toenails. Dr. Tina will treat all the toenails on the affected foot, whether or not there are visible signs of the fungal infection in the other toenails.