Everyone has different answers to these questions, which drive our acceptance of risk and how we choose to integrate
possibly a sacrificial bird from one's own flock. Wow.
A lot depends on how old the 'new' bird is. Where are you getting her from? How do those people keep birds.
Truthfully, I have kept a flock for decades. I enjoy growing my own food, and have a pretty realistic viewpoint of life and death. I have butchered my own birds, and I have lost them to predators. I have added birds and hatched out birds.
But the reality for me, is I do not have a valuable flock of birds full of rare genetics. I know several people who like me keep an egg laying flock for eggs. They and I enjoy keeping chickens. They don't go to auctions, don't do chicken shows. And often times we have exchanged chickens without quarantine and done just fine. In a way, their birds are in quarantine from my birds in their original set up. I tend to think that healthy birds look healthy. Am I risking it? Yes, but it is a risk that I can live with.
So, if the person you are getting the bird from, just have a small healthy back yard flock. AND if those folks don't go to chicken auctions, and add those birds exposed to who knows what, or go to poultry shows - all where birds could pick up a possible disease. If they don't do those things - well their birds are probably as clean as your own.
If you have visited their flock, gone home and tended your own, well there is a good chance you have already shared germs.
So yes, I just added 3 pullets to my flock and nobody died. Had anyone got the sniffles, I would not have treated, I would have culled quickly. But no one did.
As Funclucks states, is it a real risk? Yes, but we all have different tolerances. If you would go into a state of decline over losing a bird, do not add birds, except as chicks - they are considered almost 100% disease free.
Another advantage of adding chicks, is then your birds are not all the same age. Always a good thing in a flock.
Mrs K