The quote function did something weird, but I think I got what I wanted to reply to here. Anyway...
Amprolium (brand name Corid) is not an antibiotic, but a thiamine blocker, which is why it doesn't do anything to treat, well, pretty much anything beyond Coccidiosis. It works well and is a more gentle alternative to Sulmet, which is harsh and can cause a continued bleed in the gut even after Coccidiosis has been cured. It is reccomended that Sulmet be used only if Corid has failed to cure coccidiosis. My first aid kit article (linked in my signature) has doses for both Corid and Sulmet if anyone needs them.
Most of the questions on Coccidiosis have been answered above, but I wanted to add something to consider in regards to medicated feed.
Using medicated feed is a big controversy and everyone has their reasons for and against it.
I have never used medicated feed and I always recommend against using it, myself. The reason is that I have now heard from more than one source (one a general statement about it, the other a person that had actually gone through treating after it happened) that
the cocci in an area can become immune to amprolium if given in low doses, which is
exactly what medicated feed is. The best route if you want to prevent an outbreak of coccidiosis is to plan ahead for your chicks' first exposure to the outside world.
Cocci are most active in wet soil. In dry, dusty soil, they become inactive. This is the optimal time to expose chicks to the soil, either by taking the chicks outside or by bringing the soil to them. What I do is I dig up a chunk of sod or some dirt for the chicks to play in from a dry area on the property, within the area that my hens roam. This not only exposes them to anything else they might encounter after being introduced to the flock, but it also exposes them to the inactive cocci in the soil and allows them to build up an immunity naturally, without risk of the native cocci becoming immune to amprolium. Of course, if you have chicks at wetter times of the year, you can always just dig up some mud and leave it somewhere to dry before exposing them to it.