Earlier quoted context omitted.
Out of curiosity, do you genuinely believe there is a citation out there relating to the treatment of chickens on this farm today vs. treatment of chickens on 10000 small household farms 100 years ago? It comes across as though you are demanding impossible standards of evidence here in order to cut off a side of an argument you don't agree with, and don't want to begin to consider? "Citation needed" is not a universa…
I wouldn't be surprised if there was a paper that compares traditional methods of raising chicken to modern ones. But for the purposes of Internet discussion I would be satisfied with a convincing anecdote.
Bird flu has been a big problem historically because of the large amount chickens in urban areas, although it had different names then. It's still a problem now, but it's now only spread by migrating birds.
When it comes to diseases. Do you think 10.000 amateur farmers know all ins and outs of chicken diseases as well as the professional chicken farmer?
And do you think they are more exposed to diseases in a farm with hygenic corridors or in someones backyard?