I've been doing a lot of reading on the MG and Coryza. While these are a couple of the dreaded nasties that chickens can get, it isn't always a death sentence. Like I said in an earlier post, if the eggs do hatch, and there is a high probability that they won't, they will have resistance to it. Cool huh? You can grow your own flock from your birds if you have a rooster and call it good. Now on the other hand...if you wanted to introduce new birds, and who doesn't, I would say there are a couple of routes you can take.
The first would be to hatch eggs that you get from someone else who has all the tests done on their birds. Can show without a doubt that none of their birds have ever been exposed, etc.. You would still have to keep these hatchlings seperated for a long while until they can build up immunities to what you may have there. Another thing is to buy your chicks from a hatchery and either have them innoculated against these or again, keep them seperated from your main flock for a while. They still may end up with the same thing but you can always treat with the Duramycin-10. The trick is to catch it really early and treat immediately. This is a case where an antibiotic is your friend.
I'll tell you this, my sister and I got our chickens at the same time. She had issues very similar to what you are going through. Looking back I now realize it was probably MG. I had her put her birds on a 14 day course of Duramycin-10 and she has never had a reaccurance in the almost 2 years she has had those birds. Lol...another thing, she put in 24 eggs in her incubator figuring they wouldn't all hatch, guess what, they are all fertile and she is wondering what she's gonna do now! LOL!!!! So much for hatchability. This is your life now and if you take the proper care of your birds, you'll do just fine. It's still a sad thing to have to learn but you deal and go on. Your birds don't know their sick and will do chicken stuff just like everyone elses. Just go forward now and learn about biosecurity and what you need to do.