I think your whole flock should be considered carriers (unless the sick ones are only the new ones and none of the new ones have mixed with your old flock yet, in which case your old flock *might* be ok). My girls got sick after a friend who also raises chickens came to fix our backyard fence... He had been working in his yard, came to mine without changing clothes and boots and only told me after that he had been dealing with sick hens

. I personally don't see a reason to cull if you're not in the business of selling birds. Sounds like you take excellent care of your birds so they are unlikely to be continually relapsing, and there few guarantees that your next flock will disease free forever. Just be careful about visits with any other chicken people