We used to know someone (acquaintance through work, not a friend) that raised and sold fighting roosters and was quite open about it. Whenever someone brought up what a horrible thing he was doing, he'd laugh and say, "I just sell roosters. I don't know or care what they do with them after they're sold."
It could easily be spread by selling infected birds and moving them around. Since it's all under the table anyway, no way to know who is doing what, how many birds are coming and going, or where the birds end up.
I'm not blaming anyone, but having known this one person I could see how it could happen.