Mary-- while I don't have an answer for you, I would like to adda question.
If birds do not show signs of infection and others do show signs of infection but recover-- are both groups worth keeping as breeders? Or is the line drawn at Very sick and needs nursing and antibiotics to recover?
Was it the bubonic plague in the dark ages where about 1/3 of the world population either died or survived. Among the survivors many never even got the disease despite being surrounded by the disease and the dying. THese are the people that went on and repopulated passing on their innate immunity thru genetics.
Great point. If chickens need antibiotics to live and thrive, can one truly say they are heritage quality birds? Or even be considered naturally hardy at that point? I've often asked this question or a similar version of it on this forum but have always gotten the answer that the chickens are given antibiotics to get over an illness just like a human would need them to get over an illness. But, is that getting over an illness or is it getting through an illness? To me, one is a crutch to help a weak immune system limp along until it hits another rough patch and needs the same crutch... and the other is walking through the fire and coming out on the other side stronger. No crutch required. Survival of the fittest and rightly so when speaking of the animal world and development of strong, heritage quality livestock.
Unfortunately, nowadays no one has any interest in developing heritage quality humans with a natural hardiness either...and that's such a shame as the world grows more filled with stronger, more hardy pathogens.