So I have some chicks from a breeder that is NPIP certified. They range in age from 2-4 weeks old. I have been keeping them inside in a large tote until they were ready to go out to the brooder, and I wanted to keep an eye on them for disease. I have been hatching chicks of my own and had some chicks picking on some of the smaller chicks so I put a few of my oldest chicks into the brooder with the quarantined chicks. I honestly didn't think a thing about it. At that point there was no sign of disease. I have had the chicks from the breeder since the 11th of March. A few days ago the 3 chicks (that I hatched here) that I put into the brooder with the ones from the breeder started drooping up and acting like they were cold even though the brooder is warm. They have not been outside since they hatched. One died last night, and another acts like it is dying today. She is gasping for air, shaking her head at times, and is droopy. The 3rd chick is a little droopy but doesn't seem as sick. The feed I am using is Dumor medicated chick starter. I have been giving them Corid since yesterday just in case. Today I noticed one of the chicks I got from the breeder is sneezing/coughing. There really is no other symptoms in that group that I have seen. All good appetites, vigorous, no droopy chicks in the bunch. But why are the chicks I put into their brooder all of a sudden sick? They have all been in a draft free, warm brooder. I am terrified of bringing something horrible like Newcastle or Bronchitis in on my chickens. Should I cull the entire bunch?? Any advice? I have some Terramycin water soluble antibiotic powder I may start. Edited to add: when the chicks were shipped one was DOA and another was droopy. The droopy chick died the next day.