Medicated chick crumbs contain a coccidistat, several are available. These aid in the prevention (NOT treatment) of coccidiosis. They prevent the replication of coccidia within the intestines. However, as I tried to explain in the essay above, the control of coccidia is a balancing act between the level of environment challenge and the resistance of the chicks to infection. The coccidiostat in the food is only one component of this protection. Imagine a traditional set of scales with the chicks resistance on one side and environmental challenge on the other side, on the side of the chicks resistance there are many factors, such as the natural barrier to infection provided by the lining of the intestines, the chicks own immune system, and in this case, the coccidiostats in the food. On the other side of the scales is the size of the environmental challenge, so, regardless of any medication, it is always possible to see any disease in the chicks if the environmental challenge is large enough. Also, some coccidiostats will reduce the exposure of the chicks immune system to the coccidia, which will reduce the development of the chicks own immunity. In a commercial setting, coccidiostats are sometimes used in pulses (eg one week on, one week off) to allow the chicks immune system a controlled amount of exposure to the coccidia, and therefore a natural immunity to develop, however this can be really complicated to impliment, so I would not advise it without specialist advise on an individual flock basis.
I hope this explains the problem.
Will