Try not to panic, even though you've been given such doom and gloom scenarios (sheesh!) It is not ALWAYS fatal and doesn't always wipe out an entire flock. There are several strains--some worse than others.
There is hope--You just need to act on it as promptly as possible, with the medications that are available to you.
So that you know what you're looking for: There is a medication named Corid (Amprolium-a more concentrated dose than what is found in medicated chick feed and comes in powder or liquid) or another called Sulmet (which, I've been told is harder on the gut).
I only have access to Sulmet, so that's what I'm using in their water --2T per gal. water for 2 days, then 1T per gallon for the next 4 days.
I agree that there's no point in isolating the chick, especially at this point, if the others have had access to its droppings already.
Take it from me--someone utterly meticulous about brooder cleanliness and dryness--anyone can get it. I have had my 5 week-old chicks outside only 3 times--one day for 1 hour, another for 2 hours, and this past Saturday for 4 hours.
Never been any other chickens on the soil before, and they must have picked up cocci. Unfortunately I had no way of knowing how prevalent cocci might be in my suburban backyard.
One pullet passed bloody droppings yesterday. Also, I had them on medicated feed from the day I got them (thank goodness!)
At least mine are all active and have appetites, so I've either caught it early, or it's a mild case (possibly thanks to the medicated feed).
Also, Pro-biotics (in the form of yogurt or kefir) help the chicks' gut while they're on the medications. It helps keep their intestinal flora balanced. My chicks have had a daily ration of yogurt for several weeks--I'm wondering if that has also helped lessen the severity of the outbreak.