I would like to comment & think out loud in regards to the person asking (in the thread that you posted) about how hen hatched chicks seem to contract less illness and seem healthier.
Now, I was thinking about this today as I went back and forth between my pen of incubator hatched and human raised chicks, and my hen who is caring for about one million little fuzz balls. Yep, those little hen raised babies seem more vigorous, friendly, and I haven't yet lost a chick under a hen.
So, that got me thinking .... in people, we know that babies raised in orphanages where they have minimal human contact often don't thrive well, and can develop a bucketfull of issues with social interaction, normal development, etc. We now also know that babies that are raised with lots of 'skin-to-skin' time with their mom and dad thrive better well into adolesence. I did a research paper on "kangaroo care" as it's called with humans last semester, and the benefits are amazing.
Now, I wonder if that is what we are seeing with chicks that are brooded under a momma hen? Yes they are exposed to more, can probably gain immunity that way, etc, but I wonder if there isn't something healing about a momma's touch