@ATH
In the case I just dealt with, "patient zero" was clearly infected by Beatles or a non-mankind related Factor. The tree had split in a storm and had never been trimmed because it was near the edge of a forest.
The next year half a dozen others were dead or dying.
I waited to take the trees down on a non windy day and pushed it up into a pile in as large of pieces as I could to try and prevent a lot of excessive sawing while staying away from nearby trees. I then let it on fire. By the next morning it was pretty well gone.
Most of the trees I see that get it first in an area are either already weakened from bugs and/or storm damaged.
I have seen a few recently that had been trimmed and were perfectly healthy and it looked like somebody turned them off like a light switch.
I recently moved south from Kansas City to Pittsburg Kansas. I have seen some of the worst hack jobs down here on trees that I have ever seen in my life. I saw two pin oaks a couple of weeks ago that had been "Topped" which is bad enough but this was the worst hack job I've ever seen perpetrated on two trees and people think it's normal down here.
Southern culture down here does not treat trees very well.
I think the main transmission by people is just wounding trees in general allowing beetles and other bugs or possibly even birds to spread it.