Cocci...coccidosis is a disease that poultry can get in their intestines that often kills them. IIRC, chicks under the age of 16 weeks are most susceptible. Chicks raised by a mother hen will naturally acquire immunity (or stronger immunity, perhaps not absolute) by being in what we would consider an unclean environment. They eat all sorts of stuff off the ground and are exposed to cocci...they build immunity to it naturally...or not. If you try to raise your chicks in ultra-clean conditions, you're not doing them any favors. Raise them outside, if possible (with heat of course). If that's not possible, bring them a clump of grass complete with dirt after a week or two and let them tear it up, it's hilarious! If you do raise them indoors, of course you don't want them running around in their filth too much and introducing them to outdoor things (leaves, grass clumps, dirt, gravel, etc.) Give them probiotics, such as yogurt or homemade saltless sauerkraut to boost their gut health and overall immunity. I'm not 100% sure about chickens, but something along the lines of 70-80% of humans' immune system is in the lymph tissues in and around the intestines.
But like ChickensRDinos said, it's personal preference. We don't do medicated feed largely because we don't think that pharmaceutical medicines are the best first choice. I would use such medicine if it might save a life, but not to prevent a disease. That said, we've never medicated any of our birds but I do have Corid on hand should signs of cocci show up in an adult bird. By signs of symptoms, it's likely too late, but I would try the Corid to see if it could help. For prevention, good nutrition, clean air and appropriate introduction to various pest and diseases. The best advice I have heard on worms is don't treat unless you have sign of worms. Ask other chicken farmers around you if they have ever verified a worm problem or not. Some folks worm their folks whether they need it or not...I don't agree with this philosophy, but its their business, not mine. Some folks say that worms are a natural part of a chicken's life and it's only if they get out of hand do they become a problem. Supposedly some foods like pumpkins seeds, crushed red pepper and garlic have some effect on controlling worms (but not clearing them all). We feed these foods occasionally...heck it's good food rather than some questionable toxic medicine.
Vaccinations, well that's a hot topic in the media right now. People are furious on both sides of the issue. If we ever have kids, we will certainly look at the vaccination issue more closely, but for now, for our chickens, we don't vaccinate. And Marek's is known around here...our neighbor with 100+ chickens about 1/2 mile down the road lost 70% of her flock a few years ago to Marek's. But 30% survived (with presumed Marek's immunity) and those birds then became her breeding stock...and she still hasn't vaccinated. It's called the "natural selection" method, aka, survival of the fittest. The wind sometimes blows in our direction from her place and we have been to her farm several times, so there's a pretty good chance we have brought Marek's to our own flock. So far, so good. Perhaps they have the immunity.
