it is most likely entering the flocks via people, on shoes or clothes. Once in a flock, it is 99% lethal to the whole flock.
Lets take the disease vector one step further.
You have a whole flock infected and dyeing. Every person and piece of equipment moving the infected birds are now exposed and carriers. Every person's vehicle and everything they touch and remove from the farm, like keys, pens, sunglasses, paper and hats, are now infected (these are things most people forget to disinfect and touch). The vehicle and tires are now infected and spread the infection as they leave the property.
3 miles down the road, they pass someone walking to their mailbox and on the return trip step onto a piece of fallen debris from the vehicle. They are now infected and carry it to the feed store on their shoes. everyone at the store now is infected if they step in the same spot as the person who carried the infection into the store. they now carry it back to their farm.
I was part of the hospitals infectious disease response team, so I do know a thing or two

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So the USDA team will de-populate a farm and bring in portable furnaces to incinerate the flock on the spot. They will go inside the barns in protective gear through an airlock between the barn and the door to the furnace and place all the birds in the incinerators. Once they enter the airlock again their gear will be burned and they will enter a shower area and clean up and dress in clean clothes. once they leave the property they will go through another shower/cleaning cycle before they can leave for the day.
The bedding will be doused in a chlorine solution that is at least 50% strength. For reference household bleach is 10% chlorine solution. After it is fully saturated it will be loaded into sealed connex's and taken to a larger incinerator to be burned.
All equipment will be submerged in the chlorine solution, anything that can not be cleaned will be removed and taken to the incinerator with the bedding.
Overkill? Maybe, but take the above scenario and how quickly a disease can be spread by not going to the extreme. Now lets take it to a paranoid level but one that is within the realm of possibilities. The feed store that the infection was carried into, happened to be visited by a worker from a major hatchery like McMurry and the infection was carried back to their farm and infects their flock. That unlikely but possible event would be hard to recover from. Or how about the person works for the hatchery that supplies birds to a hundred farms and hatches out half a million chicks a day. Now those chicks get infected and the disease spreads to hundreds of farms over multiple states.
Those scenarios is why the USDA will not leave the disposal of a depopulated farm to the owners because they can and do cut corners.
And this is long enough as it is LOL