I'm one of those risky people and use a different approach to insuring good health in the flock, a more holistic approach. I get strange birds in each season to use for meat purposes, some from known sources and most are not. They free range with the rest of my flock for awhile and there is no quarantine period prior to that...they get dusted with pyrethrin, castor oil on the legs and combs/wattles when they arrive and are set free. Been doing it that way for years and I've never had an illness in any of my flocks. I don't vaccinate nor feed medicated feeds here, nor do any biosecurity measures either.
I don't worry about introducing new birds into the flock, rather the introduction of any of their specific germs most likely helps to build antibodies in my own flock. I work towards strong immune systems here, culling sometimes twice a year to get the best and most healthy for breeding the next year. I depend on that health and those immune systems, on the free range on pasture that is not overstocked, on fresh air, sunshine, good gut health and also on, ultimately, God to keep them healthy.
It seems to work and has done for the past 40 yrs. I've had people call that luck, but for all the work and attention to building a strong flock that's put in here, I'd say calling that luck is like calling an Olympic runner lucky when they win.
Unless that stops working well, I don't think I'll be changing anything and will continue to take those risks along the way.
Journey, it might be something worth considering~ a more holistic approach~ as there is simply no way to avoid a flock's exposure to environmental vectors of disease.