I first encountered it in 1990, purchased 6 Boykin youngsters from a fella in Liverpool and 3 of the 6 showed signs of what we now call y/b/s. At the time I talked about it to old hands asking what could be wrong with them, got the same response from all ' something they must've picked up pecking around'. The following year I bred from these and their youngsters went down with it along with a few of my own. I came to the conclusion which I still believe to this day is that once a pigeon has contacted y/b/s it becomes a carrier, passing it onto their young where it remains dormant in them until put under stress. No cures or preventative treatment for it, just have to accept it and manage it when it flares up.