Hi. Please, don't take this as an argument. I am just going to talk about my tank and my experience. Most of the fish that died in my tank before I used a skimmer died from ick. That's why I started using chemicals like Kick-Ich, Rally, RXP, garlic, etc. None worked and fish kept getting ick and dying within a month. It wasn't until I added a skimmer that the fish stopped dying though the ick remained. The only thing that changed was the addition of an aerating device (skimmer). I have pictures of fish with ick and it isn't pretty. But with the skimmer, the fish survived and fought it off on its own (not surprising since ick affects the respiratory system I've read). Seth came over last night and mentioned why does my fish have ick spots on them and I told him that I've always had it and can't get rid of it completely. I told him that I've had those fish forever and ick is harmless.
Moo even got ick after I introduced her to the tank. Yes, Ick can kill in certain conditions, but fish should be able to fight it off in normal conditions, even impossible-to-keep ones.
So this is my experience with diseases in my tank and it works for me in my tank under Vancouver conditions.
As for quarantining, I don't because I only have a small tank and isn't worth it to set a Q tank up and I have no space.