It is quite amazing how some species are susceptible to very specfic ailments and seemingly ignore other diseases while everything in the tank dies around them. Reports indicate fish building up immunity and tolerance to some ailments.<a href=showthread.php?s=&postid=9965634#post9965634 target=_blank>Originally posted</a> by JohnnyW
I'm not the most experienced person around, but I would think that the other fish in the tank would show some sort of symptoms if a disease is present, wouldn't they?
Diagnosis is tough at this point, just assume a bug is in there, and avoid adding new livestock for a few weeks/months to let everything settle out recover, etc.
Best of luck.