when i had some cocci here, it was brought in by some OEGB i shpped in. it was spread to some d'uccles that were penned beside them. i ended up culling all that were sick.
but i firmly believe in that there is good in most all things. and that includes anti-biotics, meds, and the such. i rarely use any of them but there is a time an place for all things. i dont believe in the all powerful preventative measures that some like to tout. i feel that genetics play more of role in health than most like to give credit for. and thats in all animal husbandry, and even in humans. you breed for strength and you will generally have strong individuals. both man and beast. even still some individuals are stronger healthier than others. once again genetics.
with that said, i will use medicated start and grow for most all biddies, just to get them started. ive raised thousands this way. seen the positive results to get them up and running. it has very low doses of amprol in it. this will help with cocci. if its a bad case of cocci, you may have to go with something stronger, sulmet does a wonderful job. but being a sulpher type drug it can be rough on the kidneys. so you dont want to use it but sparingly. sometimes a product called corrid will help. there is like 7-9 varieties of cocci that are known. i cant remeber it all now, been to long since i cared that greatly about the health of fowl. some drugs work on some varieties some dont. but ive had the best fortune with sul-met. you can follow directions on the bottle or go with 3 days on, 3 days off. usually one time on will do trick, perhaps 2 sessions at times. any more than that, they aint worth saving. no matter how valuble they are. they will get better but the damage is done internally, not from the sulmet or corrid but the actual cocci. after the fact you need to put good bacteria back in their guts. yogart is the easiest cheapest way that ive ever used. there are others but for most here, thats all thats necessary. at times a sickness will take the "body" off fowl. they will be puny, no breast, etc. bay-tril is a good drug to help them recover in this instance. all these drugs mentioned are available at most any/all feed stores.
now,,,,i dont recommend personally that any of this to be done but in extreme situations. extreme as in value of the birds. some birds are worth hundreds of dollars. a little bit of medicating is of absolute necessary when your dealing with this kinda money. show stock, proven breeders, etc. i wouldnt waste my time or money on drugs for the barnyard types or poor quality stock that most can buy from feed stores, hatcheries or what most are selling at swaps or on craigslist. these are easily replaceable, for just a few bucks. drugs arent cheap.