When we bring new birds in we quarantine for 2 weeks and then add a "sacrificial" bird from our existing flock and keep them together for 2 more weeks before putting them all together.
Having a totaly closed flock is good in alot of ways but not always practical. Let me use a friend of ours for an example. They are clean freaks, you could find a speck of dirt in their house at any time, they use anti bacteials, bleach, etc etc etc. They are scared to death of catching something the kids get disinfected if they go outside. You know what? they are always sick with something. Our kids would run and play, get dirty, eat dirt, fall down, get cut - just be kids and they are healthy as can be. I'm a firm believer in being exposed to this and that makes the immune system stronger.
Getting back to chickens it's the same thing, a totaly closed flock means they haven't been exposed to anything and could be open to catching everything. I'm not recommending running down to your local sale and getting a creepy bird but sooner or later you will have to add new blood.
Steve in NC