it sounds like its Paramyxo? judging by your discription if ones got it they will all have it just some will show the symptoms like losing ballance head kneck twisting as long as they eat and drink they will over come it jab them and keep multi vits in the water and they will come ok but still get them tested and before any one knocks this i had it many years ago with my y/bs and let it run its corse
and with in a fortnight all clear i culled 2 that was all and some of the best birds i ever had where bred that year