Our one and only case of Coccidiosis, a Cream Legbar chick at 4 weeks, was pronounced beyond help by the vet. At that stage she was pooing just blood and was hunched over. In desperation I bought a tub of probiotic natural yoghurt, filled a syringe with it and filled the little girl's crop to the limit. Next day she was fine! It repeated at 5 ½ weeks but the same treatment saw and end to it. She lived another 6 years.
We now give all our chicks a spot of probiotic yoghurt at day two. They are reared inside on unmedicated crumb and go out at 6 weeks. We have never had another case of it.
However chicks outside under a broody get medicated crumb. They also get a spot of yoghurt when we can catch them. It does inhibit their growth though, so at 8 weeks they are the size of the internally reared 6 week chicks.