Once the affected birds recover, they should have resistance to whatever strain(s) it is, and not get sick again unless they are weakened by something else. Coccidia are everywhere, it's a global issue with chickens. Young chicks are most at risk as their immune systems are not completely formed and they have never been exposed. It can be spread in droppings, so keeping things as clean as possible (within reason) and dry (no spilled or leaking waterers), and keeping feeders and waterers free of droppings can help. It often happens when brooder raised chicks are put outside for the first time. To reduce the risk of this I put a shallow dish (like a plant saucer) in my brooder filled with dirt from the yard (not from the chicken run) so they can dust bathe, and scratch and peck in it from the very beginning. That exposes them early on to all the microbes in the soil so they can be exposed gradually, just like if they were being raised by a hen. The saucer makes it easy to dump and clean and refill each day (they will make a mess of it - chick party!). This has reduced my incidence of illness a lot. But I always have the meds on hand just in case, so I can treat right away if there is illness.What’s