@Fluffnut, there are ways to raise their immunity so they don't become easy victims to any Tom, Dick or Harry bacteria or virus that wants to prey on them. From my experience garlic really helps. I have garlic growing in a pot and I regularly offer it free choice. Who eats most of this green garlic? The one prone to bumblefoot. I didn't have time to treat her bumblefoot and when I finally decided that I shouldn't postpone it any more, the bumblefoot was GONE. Spontaneously! I couldn't believe it! It had been a superficial infection, from wet feet, but a real infection nonetheless , red and hot under the scab and with loss of appetite.
Also free choice: oregano. You know about oregano the savior of poultry, right?
Number three : mushrooms. Whatever cooked mushrooms I have, I mix some in their feed.
Only scratched the surface here, hope this helps.