Sounds like a nail fungus to me. Fungus loves dark, damp places, so toe nails are the perfect host! To get rid of it without using drugs you can soak them in a 50/50 concoction of vinegar and 3% hydrogen peroxide. It will take weeks. Keep them as dry as possible and out of shoes where your feet get extra damp and the fungus could be living. A very, very diluted bleach and water soak can also work, but only for a minute or so. Don't over do it!
Sunshine is great for killing it, but nail polish will prevent any of this from working. Oh! Just saw you are a man! You probably won't be wearing any nail polish...
consider sandals this summer to let them enjoy the sunshine and fresh air, though!
You do want to take care of it, though, because left untreated sometimes people lose the entire nail...
Yes, there can definitely be an underlying systemic reason for the fungus. Anti-candida diets (cutting sugar and other bad carbs) are recommended by the alternative doctors, as well as consuming more alkaline foods like green leafy veggies and apple cider vinegar (take a shot or so a day mixed in a large glass of water); healthy things to do for all of us! I've also read that taking virgin coconut oil (VCO) internally, as well as rubbing the nail area with it after they are dry can be very helpful. VCO is a natural anti-fungal among other things.
There are over-the-counter products as well, but they won't keep it from coming back.
Mom to 17 year old son diagnosed in June, 2007.
Omega 3s, digestive enzymes, probiotics, vit. C, calcium, vit. D3, a good multivitamin and SCD legal yogurt
Started The Maker's Diet in Sept. '07. Incorporate Specific Carbohydrate Diet (SCD) recipes, too.