You may be interested in my chicken rearing experience.
I have been breeding Japanese Bantams and Cochin Bantams for over 12 years.
I have never vaccinated
I have never de wormed
I have never given antibiotics
I have never given vitamins
I have never given oyster shell or chicken grit
My birds are all in top condition.. plump, soft glossy feather, red combs, perfect eyes, smooth legs and feet, friendly and placid and easy to go broody and great mothers and fathers to their chicks. They are very sought after by others into the hobby, and my birds have a very good reputation in my area!
I rarely to never loose one of my birds to a disease.. most die of age related problems.. many of my birds are over 8 years old and still laying and hatching eggs.. I still have 2 of my first birds.. which must be getting on for 13 years old.. one of which survived a horrific dog attack and starvation when she was lost in the forest for over a week when the dog left her to die.
In the first few years I had many problems with disease..
fowl pox, wet and dry
scaly leg mites
fungus on the combs and wattles
and other things I have long forgotten
I lost a few birds to these diseases. My treatment was to keep the sick birds in a quiet warm and stress free environment.. give them nutritious foods, put apple cider vinegar in the water... and wait. Most recovered.. a few died or if they were suffering I would cull them.
Over the years I lost fewer and fewer birds to diseases. I would cull out any birds that were aggressive to each other or me.. I would cull any that were bad parents or brooders (such as killing or injuring their chicks or others chicks, or egg eating etc.)
My roosters are so placid.. gently, mix with each other without fighting and don't over breed the hens. The roosters are also great fathers.. seeming to love to socialise with the chicks.
The hens are all good mothers and show little to no aggression to other hens chicks.. allowing them all to mix into big groups and the hens share nests, brooding and care of the chicks.
Over the years it has been survival of the fittest by nature.. and the culling of birds with bad temperaments by me.
My birds are free range during daylight hours.. living in an orchard.. they get all the greens and insects they need by themselves. In the evening they go back to their coop and I just lock the door.
The only feed I give them is a general flock raiser.. I give this to all ages. They also get some wild bird seed (as my fancy pigeons also share their coop).
I give them back their egg shells to eat.. and they eat snails and their shells, and grit when they are free ranging.
I put apple cider vinegar in the water 2 or 3 times a week. And they get clean water 2 X a day.
The only chemical I use is a dog flea powder.. which a shake into their dust baths occasionally.. although I have never seen any mites or lice on the chickens feathers or bodies.
I cured the scaly leg mites using cooking oil on their legs a few times. (here again I notices it was certain lines of birds that's seemed less resistant to the scaly mites).
That's it.. so I can say breeding for resistance really works for non commercial flocks.
PS. I rarely buy in new birds.. but on occasion, when I have, I noticed how they have virtually no resistance to common diseases.. and some will die. An example is dry fowl pox.. which all my growing chicks get once they reach adulthood.. but they only get a few lesions and don't get ill and all recover fast. a new bird will get the same strain of fowl pox but get many lesions and will seem to be suffering... and take much longer to recover. I even had one new pullet only get one lesion on her comb and she was dead 2 days later!!!
I think the chicken breeders are over medicating their birds, and also keeping them indoors or in very bio secure environments.. so they have little to no resistance to diseases out in the real world or free ranging. However, when I cross breed them to my birds the resulting offspring seem to inherit the resistance from my stock.
I know of other breeders who vaccinate all their birds, almost constantly have them on anti coccidiosis medication or some kind of anti biotic.. and their birds are skinny weak looking things... and once they come off the medication the go downhill fast.