I chose to raise/breed silkies who are more prone to vitamin deficiencies and diseases like Marek's.
In nearly a decade, we've lost one to liver disease (vet affirmed), and other than treating one with a persistent URI (upper respiratory infection), a few other losses were just happenstance. I don't count the several splayed legs and neurological issues over the years, or other physical ailments some have come up with. Those are few and far between, and we just treat them.
Sticking to clean, secure, well-ventilated coops, fresh water, good feed, healthy treats to a minimum, and plenty of room per bird in the coop/run are the basics. Occasional probiotics and vitamin boosts can help.