Kim, I don\'t think anyone can say for sure what the problem is, but here is one possibility. If incubation is not just right, chicks can hatch with unhealed navels. If there is bacteria or other germs in that warm, moist environment they are hatched in, they can get an infection in the unhealed area, and that spreads into their systems. It can take anywhere from a day to a week to finish killing them. If they make it through the first week and then die it will be from something else. I don\'t believe Marek\'s would have that kind of effect on newly hatched chicks. Try disinfecting the hatching tray thoroughly before the next hatch, and see if it doesn\'t make a difference.