Toenail fungus is a common condition that begins as a white or yellow spot under the tip of the nail. As the fungal infection goes deeper, it may cause the nail to discolor, thicken, and crumble at the edge. If left untreated, the fungus may spread to the other toes, the feet, or even the fingernails. Those who suffer from chronic diseases, such as diabetes, circulatory problems, or immune-deficiency conditions, are especially prone to toenail fungus. Because clean, dry feet resist disease, proper hygiene is the first line of defense against fungus. Disinfecting nail-cutting tools before use can also prevent nail fungus.