Well sounds like you found the culprit. Cocci isn't contagious per se as its in the soil and always in the soil. Often if a bird get's stressed for some reason, if there is a big change in the weather, or if the chicks were kept in a too clean of a brooder without soil contact for most of their lives prior to being let out to the soil, can all be things that take part in older birds getting cocci. Best of luck!