Hi lads, I thought I'd throw this in as my conditions are probably unique and may be of some use to your theories, I haven't raced since 1986 but have always had at least 40 birds, now about a 100 , We were vaccinating against PMV back then but I can honestly say I've never had PMV, I don't vaccinate now, and the only thing I know about YBS is what I read on here, I only race One Lofts so nothing comes back here, I also rarely see strays being remote, I'm on mains water and feed on Bucktons mixes plus the usual grits and minerals including Matrix, I think Walter is on the right lines thinking the link is regarding the PMV vaccination, Hope this is of some help. Bill