It may be a flesh "eating" infection.
Try looking through these sites:
http://www.aquatronicsonline.com/hobbyist/hobbyist3.htm
http://www.aqualink.com/disease/sdisease.html#ich
http://fish.mongabay.com/diseases.htm
http://www.wetwebmedia.com/AqBizSubWebIndex/fishdisho.htm
http://www.fishpondinfo.com/health.htm
Some background on your tank would also be helpful. Answer these Qs to ensure we have as much information as possible about your ailing fish and its environment:
1~Please describe what is wrong with the fish to the beset of your ability (i.e. cotton like growth, bloated, etc.). Are the scales gone? What color is that area?
2~What are your tank parameters (ammonia, nitrites, nitrates, temp, pH)? Please give exact values.
3~ How large is the tank? How long has the tank been set up?
4~What type of filtration are you using? Please give the name and number (i.e. Fluval 304) and amount of
gph if known.
5~How many fish are in the tank? What kinds of fish are they and what are there current sizes?
6~When is the last time you did a water change and vacuum the gravel? How often do you do this? How much water do you remove at a time?
7~How long have you had the fish? If the fish is new, how did you acclimate it/them?
8~Have you added anything new to the tank--decor, new dechlorinator, new substrate, etc.?
