Article in the Guardian includes:

When a bird is bred so that all its energy goes to the production of meat or eggs, “something has to give”, says the ASPCA’s McMillan. “The science indicates that a bird’s immunity goes down.”

As Greger puts it: “There is an inverse relationship between accelerated growth and disease resistance, which means faster-growing birds are more susceptible to illness.”

While the USDA terms this outbreak “a wake-up call on biosecurity”, the idea of hermetically sealing farms, which use ventilation fans to keep birds cool, may be too difficult to enforce. “The industrial poultry system, by its very nature, is vulnerable to these kinds of infections,” he says.

It’s the system that is at fault, according to McMillan. “We are forcing birds to live in unbalanced ways, both physically and genetically,”she says. Commercial poultry flocks “are bred to suffer. We force them to live a life of misery, and from that perspective, they are going to be more prone to contracting and spreading disease. These are not healthy, balanced animals.”[/unquote]