I know for a fact that some fanciers vaccinate with vaccine which has not been kept chilled when sent out to them. This is a suppliers' problem and they should insist on the proper distribution method even if it costs a little more for the cool bag and slightly larger package. I then know of fanciers who do not store their vaccine in a fridge as indicated on the instructions. I also know of fanciers who use vaccine after its first use and outwith the two / four hour period. I also know of fanciers who use vaccine which is past its use by date. These are fanciers' problems and this incorrect use could be a cause of PMV / Salmonella arising within their lofts. Whether any of these factors encourage YBS, I do not know, but whilst fanciers use vaccines not licensed or prescribed within this country, not withstanding their indiscriminate use of medicines, and the incorrect use of the aforementioned vaccines, I do not think they are qualified to state the cause for YBS. They may have very good guesses at the cause, but they will not have scientific proof.