A quick review from my perspective and experience:
Cryptocaryon irritans is carried into the tank with fish. It causes many fish deaths in closed systems. The odds of getting it from rock or other sites are very very unlikely unless you pop in a piece of rock in from an infected tank,n which case you may introduce a cyst.
Crytocaryon can be kept out with quarantine and preventative treatment. It is very very highly likely you can have a tank free of ich with the reasonable precautions of preventative treatment and quarantine.
Some , will say they have tanks without those measures but it is much less likely. One might cross a busy avenue blindfolded and not be struck by a vehicle but it's much safer to cross at an intersection and be guided by the traffic signal. Better yet is to close the road you choose to walk on to traffic.
Once ich is in a tank it will be there for years even if no new strain is introduced or fish are void of symptoms or present only minor symptoms from time to time. Garlic and other homeopathic remedies or the "reef safe" potions and foods in bottles sold in the hobby won't get rid of it or control it.
Once a tank has ich,the tank needs to be fishless for 72 days for the cysts to expire and fish treated in qt with copper, hyposalinity or my preferred method which is tank transfer.
There is plenty of good information in this thread ; some not so good once you get into personal theories and panaceas.. If you have a problem with ich, I suggest you go to the fish disease forum. The folks there know how to help.
In the meantime this thread may be of interest:
http://www.reefcentral.com/forums/showthread.php?t=2185929&highlight=fish+acclimation+and+quarantine