Eggcessive has good suggestions...I've considered Coccidiosis as well...you might try Sulmet. That treats coccidosis and as a sulphur drug would also treat gram negative bacterial diseases such as Coryza.
I've used Sulmet when I didn't know if it was coccidia or bacterial in a "hail mary" pass to help a bird that is going down fast. Often it works. Follow instructions on label for chickens.