At first, chickens will behave normally. As coccidia multiply inside the intestines and feed on the intestinal lining, inflammation occurs. The chicken will then start to feel a bit "off". As the inflammation progresses, bacteria colonizes the intestines, and this secondary infection is what...
This is one of the legitimate reasons to quarantine a new chicken for a couple weeks before exposing it to the flock. If this is coccidiosis, and it sure sounds like there's an excellent chance it is, the entire flock has been exposed. Therefore, you will need to treat all the chickens...