A couple months ago, I moved houses. We made a coop out of a room in the barn that was used for horses or cows by the previous owner. All my chickens from my old house loved the new place and were doing just fine. Then, I decided to add new chickens. I got 4 Speckled Sussex and 2 Polish hens from a guy i found on facebook. They were about 4 months old. Then I got month old australorp chicks from another lady. Everything was fine for a few weeks until one sussex hen got sick and died. I noticed she seemed lethargic one day, then the next morning I found her dead. One by one, all the new chickens died except for 1 polish hen that survived. My old chickens were all fine, none of them died or even displayed symptoms of sickness. All the ones that died displayed symptoms of Coccidiosis, so I diagnosed it as that and began treating all the chickens for it using Corrid. Believing that the disease was gone, I bought some 1-2 year old marans about a month later. Everything was normal except just tonight, when a black copper maran hen died. I noticed she seemed lethargic earlier in the day. When it started getting dark I went out to the barn to put everything up for the night and noticed her laying on the ground. I picked her up, thinking she was dead, only to feel her kicking in my hands. I walked out of the coop and laid her down on the grass. She kept twitching, as if she was having a very mild seizure. Then, she defecated (white) and vomited (brown, as chicken feed) and took her last breath. I don't believe I am dealing with Coccidiosis anymore seeing as that the maran hen was mature. Is there any way that horse or cow manure could be getting stirred up in the coop to spread disease? Why is it not harming my old birds? What disease am I dealing with? How can I treat it?