I've seen so many people who have bought new birds and because they seem healthy, immediately throw them in with their flock or put them in a pen where there can be some contact. You MUST quarantine newly purchased birds unless you have bought chicks directly from a hatchery. Disease can take up to a month to show up in a seemingly healthy chicken. Many, many of these common diseases make a chicken a carrier for life and if your flock gets it, they become carriers for life. Some are even reportable to the state and the birds must be destroyed in some cases. Some sellers are not aware their birds are ill and some are just plain unscrupulous and don't care. I have only purchased ONE grown bird and he was in quarantine for over a month, while I consulted experts about various things. At the very least, he did have favus, a fungal infection on his comb and face and lice. I treated both and he had neither by the time he joined my flock, but it was very stressful and I'll probably never buy adult birds again. This can cause such pain and devastation, that I really wanted to reiterate this point to all my BYC family.