Yellow nails can have a number of different causes, as we mentioned, but fungus is one of the most common. In fact, roughly 50 percent of nail discoloration is caused by fungal infections [source: MedicineNet]. Besides having an undeniable gross factor, nail fungus can also be painful and even pose serious health risks to individuals with weakened immune systems. Accordingly, you should set an appointment with your doctor at the first sign of nail fungus. While over-the-counter medications are available, they're typically much less effective than prescription medications. In fact, even with treatment from your doctor, nail fungus can be difficult to get rid of. In some cases, the infection will recur, making effective treatment that much more important.
For most infections, your doctor will prescribe an oral anti-fungal medication, which you'll need to take regularly for a month or longer. In time, the infected portion of the nail will grow out and be replaced by new, healthy nail growth. Make sure to tell your doctor about any pre-existing conditions, however, because anti-fungal medications can have serious side effects for people with certain liver and heart problems. For more information about nail fungus, read Nail Fungus: Fast Facts.