as far as i understand, ,,, in the body of a pigeon [and humans] there is" good bugs" and "bad bugs" , they are continuasly fighting each other,, when the" bad bugs" beat the" good bugs " the bird becomes ill,,,, treating with broad spectrum anti_biotics kills all the bugs,,,good and bad ,, then the bird must make up more good bugs [immunity bugs] off course it can make up more " bad bugs" at the same time,,,, but treating with a specific anti-biotic only kills certain" specific bugs" , thats why a vet is recommended in asserting the proper anti biotic for the proper job,, ,,in my opinion many fanciers use, or rather mis-use antibiotics by treating willy nilly,,, even the doctors nowadays are known to be using antibiotics far too much for simple ailments, and not allowing the immunity system "the good bugs" to fight simple ailments,
if the immunity system "the good bugs" can beat an ailment "the bad bugs" themselves, the chances of this bird catching this ailment again are less [obviously things are more intricate than this , but this is roughly the gist of it,,,in my opinion