I have only had to deal with cocci twice. Both times were when I was not feeding medicated feed, it had been unusually rainy, and the chicks went outside without having been "inoculated" with clumps of grass and roots in the brooder prior to moving outside. Once was when I had some ducks in with them and didn't want the ducks to get too much medicated feed, the other was this week! I moved them out to a small pen inside the coop (wasn't thinking) and gave them some gamebird starter because it was handy.
I lost three in 48 hours. I went to TSC and got a pack of Corid (Amprolium , 10oz for $22
.) I dosed them and didn't lose another one, it works well and quickly, but it would have been cheaper to feed the medicated chick starter and remembered to give them a clump of grass a couple of time a week. Here in the south, with the heat and humidity, cocci flourishes. It is a good idea to stick with the medicated feed (or medicate it yourself, like Tom said) until they are fully feathered and have had a chance to build up their resistance to cocci.