All of my chickens are dropping dead, sometimes with symptoms and sometimes without. At times they appear to tumble off their perches during the night after having no discernible trouble the day before. (Notice I say "discernible, as all birds are masters at hiding illness to throw off predators). Others spend a few days with little use one side including wing and leg. Yet others just look punk; these either recover or die. My pens are such that anything can pass easily between them, which is what happened. It hasn't happened all at once, but after near a year I lose two per month. I gave them a five-day course of the legal alternative to Sulmet. Has anyone had anything similar happen? If I have new chicks inoculated for Mareck's, will they be immune to it when exposed to the others?