Without seeing a picture of your ill frog it is hard to say what it might be. One disease that is known to occur in frogs in captivity is called Lipid Keratopathy. It is believed to be caused by some type of malnutrition. It can be hard to recreate a frog's diet in captivity. Here is a link that has a picture of a frog with Lipid Keratopathy and information about it. images.google.com
