Poultry Nutri-Drench is a wonderful product IMO. It's a must for my chick hatching tool box.
Giving your birds access to free range, on ground that has not been treated with insecticides, herbicides or fungicides is also a must. Getting those chicks exposed to your soil within the first 2 weeks of hatch.
Deep litter in the run so they are not constantly exposed to "sick soil". (fecal laden dust bowl in dry weather, and mud pit in wet weather. The DL creates a nice black spongy compost that is full of plenty of microbes, fungi and insects that immediately process any chicken feces. All of that soil life also gives the chickens healthy gut flora, which IMO is the fundamental basis for a healthy immune system.
Fermented feed kicks your bird's nutrition up to the next level.
In the absence of a clearly defined injury, I would choose to cull rather than treating my flock with antibiotics. Take care of their immune system and disease becomes a very unlikely event.