my understanding was that Corid blocks Thiamine (the vitamin) which the Cocci bacteria or virus needs to survive. So effectively it causes a vitamin deficiency for a short period of time. The short period of time (5 days) won't cause permanent injury to the chicks but will rid the birds of the cocci. THis is why its important to not give the birds anything but the Corid and regular food (no extra vitamins/minerals/supplements/treats etc) while they are being treated or else it undermines the treatment.
After treatment, its important to then give the birds a good vitamin treatment (Nutri-drench) and some probiotics (I've read about Probios)
After treatment, its important to then give the birds a good vitamin treatment (Nutri-drench) and some probiotics (I've read about Probios)