The most common cause of yellowed, thick and /or deformed toenails is a fungal infection of the toenail. The fungus that infects the nail, is usually the same fungus that causes athletes foot. It tends to be slowly progressive, damaging the nail to a greater and greater degree over time. The infection usually starts at the end of the nail and works its way back. It usually is not painful and often not noticed until it has gotten well established. A single toenail or any number of nails can be affected. It can also occur on just one foot. Over time, the nail becomes thickened, crumbly, and distorted in appearance. Sweaty feet contribute to the initial infection process and contribute to its spread. The fungus prefers an environment that is moist, dark, and warm, which is why it affects the toenails much more often than fingernails. It does not spread through the blood stream. The infection limits itself to the nails and skin. It is often found in association with areas of dry scaly skin on the bottom of the foot or between the toes. The dry scaling skin is frequently found to be chronic athletes’ foot. It is not highly contagious, and family members are almost as likely to contract it from some other source as they are from the family member who has the infection.