Thinking it may be Cocci...

We had one chick die at 9 days and now at 4wks another one is sick. I didn't think anything of the first one having cocci as there were no signs other than extreme weight loss, not wanting to eat/drink, lethargy. No poo problems. I had all my chicks vaccinated at the hatchery(McMurray) for both Marek's and Cocci and could NOT feed them medicated feed as it would null the vaccine. Now, from reading countless posts and books, I think we may have a chick with a gut full of cocci. From what I'm gathering, cocci is something that will always be and it is better that chickens develop an immunity to it than knock it out of their systems completely. That is assuming that they are overall healthy. If this poor little one is still alive in the morning, I'm going to run to the farm store to get some meds, but my question is, should I go ahead and give it to them all, or just the sick one?? I wouldn't want to mess up any immunities that have been built up in the ones that look healthy right now, but I don't wanna lose anymore birds either.