There are several different strains of the protozoa that causes Coccidiosis. Each strain attacks a specific part of their digestive system. Immunity to one of those strains does not give immunity to any of the other strains. For them to develop immunity to one of those strains they need to be exposed to that strain for around 2 weeks.Can a chick develop immunity to Coccidia when there is no Coccidia in their environment?
Yes. It is also possible to have more than one strain.Is it possible to have an environment that does not have Coccidia?
They do not always have bloody poops when they have Coccidiosis. Some strains are more virulent than other strains. Some of that depends on which part of their digestive system they attack. Do not rely on bloody stools to tell you that they have Coccidiosis. If they are lethargic, just standing around fluffed up and looking miserable assume they have Coccidiosis and start treatment. Even if you are wrong and Coccidiosis is not the problem treating them will not hurt them.I don't use medicated feed but watch the wood shaving bedding for reddish spots. It is rare for there to be signs of bloody poop for the first month but if any is noticed they get Corid in their water.
The problem is not when they have some of those protozoa in their system, it is when the numbers get out of hand. They need some to develop immunity to that strain. Those protozoa thrive in wet soil with poop in it and in dirty water. If you can give them clean water and keep their environment pretty dry Coccidiosis is usually not a problem but some of the stronger strains can still cause problems.