I have 14 chicks that are 10 weeks old (Wheaten Marans and Lavender Orpingtons). They're in a separate area in the coop, and during the day outside, in a separate run, segregated from the older ones. They seemed to be doing great, until this week, I found one dead in the run. No sign of a fight. Just a drop of blood on the feathers at the bottom and maybe a hint of blood coming from the vent, but I couldn't be sure of that. I checked her over thoroughly and could find no sign of trauma. A few days later, another chick was suddenly down, not walking, keeled over easily but kept trying to stand, but staying pretty much in one place all day. That one seemed alert and looked healthy, standing up at times, but seemed weak, and died after a few days. I got the feeling it could be Marek's from the way it suddenly went down off its legs and seemed weak on the legs, but otherwise healthy looking. (I lost two to Mareks a few months ago, at about 20 weeks). They've been vaccinated.) I didn't think the first had any indications of Mareks. Their stools look normal. I'm now adding apple cider vinegar and Poly vi sol to their water, to try to keep the remaining 12 healthy. My question is related to giving them vitamins. I have read to give chicks Tri vi sol or Poly vi sol. I chose the latter, because it has B vitamins, which could provide support for their neurological system, I thought. But, for 12 chicks, a few droppers full of Poly vi sol in water doesn't go a long way, and it's a very low dose. Would it be good/advisable to give them a larger dose of a multivitamin, let's say an adult dose in a litre of water? I read in one of the threads that chicks can die of Vitamin A, D, E deficiencies, so I think it can only do good to supplement them a bit having had two sudden deaths. I know there's a good chance that I could lose a few more to Mareks disease, before they're 20 weeks old. I'm hoping I can build up their immunity and their neurological system and keep them healthy. I know that when they're vaccinated, about 5% can still get Mareks. I seem to have a slightly higher incidence here. It's possible that the vaccination was not done exactly right. I've questioned the person I bought them from and she seems to know what she's doing, and she has a very clean, impressive poultry operation and seems very observant and conscientious about looking after her chickens. But she's also still learning, being new to the business as of 2 years ago. The chicks are on grower mash, and some vegetable scraps and an occasional handful of hen scratch. They started out on medicated starter for the first 6 weeks. The remaining ones so far look healthy and active. Any suggestions?