As far as I understand, the parasite exists in many (most?) aquariums, and a fish with a weakened immune system will become infected easier. Similar to us humans who have white blood cells to fight off bad antigens (bacteria). If our immune system is weakened due to stress on our cardiovascular system, we may catch a cold (bacterial infection) easier.
With that said, I decided that before investing in the extra tank and equipment to set up a QT... (I live in an apartment so I don't have the luxury of having room everywhere to set up these extra systems, especially to keep them looking pretty.) I would try the advice from my LFS
of giving them some garlic/omega food, and raising the temp in the water a few degrees. (which does not kill the parasites, but will speed up the process before the parasites 'drop off' the fish.)
My fish had the ich for 2 days. I gave them the flake food, and raised the temp. The white spots left the fish within 6 hours of the temp raised, and they immediately became much more active and playful. It's been almost a week now since then, and they're still fine again. I'm sure the parasite is still in the system, but the fish do not seem to care anymore.
A separate QT tank with hypo treatment is always better, but this is what I'm experimenting with right now to see what happens. So far, so good.