I was told by the state vet that we should always practice "all in/all out" with the flock. Meaning that we never mix ages of chickens or bring new birds into the flock in any way. All new birds in, when it is time for them to go, they must all go, and then we bring in all new birds. At first I thought - "Wow. Those folks on BYC are constantly talking about mixing ages and new chicks and birds, etc. Why do they do this if it's so awful and even "dangerous". Now that I've had several weeks to let it simmer in my mind, I am more of the opinion that she was saying what she is "supposed" to say. Chickens have been domesticated for hundreds of years and it seems unlikely that farmers in days gone by practiced all in/all out. Yes? No? I know several local breeders/keepers who hatch out chicks and add them to their flock, whether it's to maintain enough for eating and eggs or just to have more. And while I'm on it, the state vet also had me all worked up telling me that we needed to cull our entire flock, disinfect, and then order chicks from only NPIP certified hatcheries. This due to a positive for MS (not MG). I was beside myself. I just could not believe what I was hearing. I started doing research. I found that MS is spread by wild birds, insects, etc. That unless you keep your flock in a building and their feet never touch the earth, they are quite likely to get it. Sure enough, I found several posts on here about MS being very common. I emailed 4 major hatcheries, all NPIP certified, and all four said they can not promise or guarantee that they are MS or MG free. And since it can be passed through eggs, well there is really no way to be 100% certain that you even have "clean" chicks. And of course, once they set their feet on the grass, they are susceptible. We have decided to keep our flock. The entire situation was very upsetting for me and I think that I am finally ready to discuss it. Curious on others thoughts and experiences.