The best thing to build immunity is exposure. Years ago large farms would expose birds to certain illnesses and have the whole flock become immune to it. Now days there are meds available for certain illnesses and you need to diagnose the correct illness to treat it. I am a firm believer in antibiotics, I treat them once, it kills or cures. Overall you have a healthier flock. You keep them too clean and your enjoyment will be your time treating them for every illness out there. As far as carriers go unless you raised them from eggs potentially all chickens are carriers, realistically. Someone suggested vitamins which is the first thing I would do. If you use antibiotics follow the directions to the letter. I raise close to 1000 birds a year and maybe lose 1 to 2 percent.