The only two pathogens I have dealt with are fowl pox that appears mosquito born and what has clinical signs of Marek's Disease. Both impacted only a small number of birds, usually in their first year of life. There has been a very strong pattern where birds affected by a given disease have been closely related. I culled promptly the sick individuals. With the Marek's, a entire family was culled over about 5 years. I no longer see the disease after the family was removed. The fowl pox is also limited in scope and impacts only a small percentage of chicks each year. If two chicks impacted, then more often than not they are from the same brood, and they will have siblings that are not showing signs.
My flock is comparatively small, closed, and multi-generational. They are not packed together and continuously stressed like in commercial production. Some losses are tolerated, even preferred to remove infirm individuals. Most disease manage methods are geared towards protecting large commercial flocks because that is where the money is. If the state (government) and commercial scale producers had their way, then small flocks would be eliminated.
