Ran across this thread today and wanted to post that the NPIP inspection on my flock was all negative. There was a great sigh of relief. I can only guess that in a large hatchery, that eggs are from many chickens (so some could be infected with pollurom and some healthy) and eggs could be combined in a single hatch and sold in groups that could include both infected and healthy chicks.
This has been a real eye opener for Phlyinphebee and me. We were careful about buying birds and eggs from what we thought were safe sources. Like many chicken keepers, my exposure to chickens was at the knee of my grandmother and great aunt and it never had the down side of any chicken illness. It was only egg gathering, and getting to throw out scratch to happy chickens.
I so wish we had a vet in our area that would examine chickens and give guidance on flock management when any symptoms occur. This forum has been our greatest source of help. Thanks BYC members!!