Someone whose idea of breeding for resistance being allowing sick birds to recover, with or without treatment, is someone I do not want selling to me or anyone else on BYC. We hear enough complaints about sick birds making the buyer's flock ill. Nobody ever does actual testing to see if the birds they want to sell are carriers of whatever they had, so there is no way to be sure it won't be setting up heartache in someone else's flock.
I am 100% in agreement with NYReds: Recovered carrier birds are not strong, they are not resistant (unless you're talking something minor like fowl pox and not actual respiratory stuff), and they will make birds in their new flock come down with the same thing eventually. Some avian diseases are like the herpes/HIV virus-they never leave the body even when the bird appears healthy. Remember Typhoid Mary?
I agree also. I would not keep a sick bird period. Injury is different, but no sick birds .... resistance or not, I do NOT want sick birds! I will, however, treat for worms, cocci, and parasites. I don't see these as diseases or weaknesses to cull for.
I am still learning, and I appreciate everyone's advice and insight on the subject.