I have a coop out in the country with a dirt floor. The coop is very dirty and not very airey. I have lost about 20 chickens. They get black spots on their comb and around their beak. Their comb begins to droop. The chickens act fine, may be a little lethargic. Whatever this is that is getting on them, or whatever disease, it appears that they can build an immunity to it. I have three chickens left that previously had the black stuff and it doesn't seem to affect them anymore and they appear healthy. If I put new chickens in that same coop, they get this stuff after a few days. I have two chickens about 20 feet away in their own coop, and they don't have any of this. And they somewhat comingle with the others, but don't go in their coop. I put 10 turken chicks in there about a week ago. I noticed some of them have the black crud starting around their beak and head, and they are dying now. I don't know if this is some sort of virus / bacteria, or is it mites or something like that. At first I thought mites, but they seem to develope an immunity, or at least the stronger ones do that survive this plague. I live in central Texas. This started last summer. We've had several hard freezes that I thought might help, but it doesn't seem to have had any effect. Any ideas?