I was worried about Mareks too. I had each one of my silkie chicks from the same breeder ( 4 of them ) go through this spring. It took about 2-3 days for improvement. They went through it almost one after the other. Now I treat all chicks water a couple times a week. Good luck
It could be a B vitamin deficiency. I use Rooster Booster brand B vitamin. You can dose the chick individually and also treat the water for the other chicks as a preventative.