Why are they on Corid? Did you start treating for cocci? I agree that I wouldn't use two drugs at the same time, but more importantly , I don't think you should mix two drugs in the same water container because drugs can have chemical interactions that may be harmful. I have read tons about IB, and it can act just like you describe. They can stretch the neck out and make a little sound like a chirp, hiccup, but actually is a sneeze/cough. IB usually affects 100% of chickens within days, and can be hard on young chicks with fatalities. Does anyone in your area have chickens from where it could have spread? I have never heard that gapeworm could spread through an egg. I would like to see that article.