For some reason I can't copy/paste into this reply box, I'm trying to provide links to more information. If you go to the top of the fish disease forum, click on the Fish Parasites sticky, then Brooklynella hostilis and Uronema marinum, there you will find all the info you need on Uronema. It is a serious problem in your display because it doesn't require a fish host as part of its life cycle, therefore, pulling all fish will not cause the parasite to die off. Paraguard will not help, the best course of action is to pull the remaining fish, treat them appropriately in quarantine and bleach your tank. Some have suggested to remove all inverts and treat the tank with chloroquine phosphate, but I'm not sure how you would then treat the inverts, basically you can't. It's a terrible parasite in a reef tank, always quarantine for at least 10 weeks to allow symptoms of such disease to become present.
Also, there's another current Uronema thread, Wound(?) on Side of Butterfly, please reference the pictures of the saddleback butterflyfish's infection to help confirm the diagnosis.
It may take some fish a bit longer to succumb to the disease, hence the reason you have a couple seemingly unaffected fish.
