I hate to be Debbie Downer, but when my Marek's hatch was about 5 weeks old they started to act lethargic one at a time. I'd give each one Corid followed with extra vitamins. I'd say six or so of the 20 showed signs of going off food and acting sluggish, all at slightly different times. By 8 weeks or so they were all doing well. Then at 11 weeks the first chick (an olive egger) that was sluggish at 5 weeks starting limping and that was the beginning of the end. She was the first pullet I sent in (at 14 weeks of age) for necropsy and it Marek's. She had sciatic never deterioration in addition to a ton of small and large tumors all over her internal organs. Good thing is she was eating up a storm and didn't seem to realize she was sick.
I didn't document exactly who was lethargic at 5 weeks, but I'm pretty sure they were the first ones to succumb to Marek's. Please do keep notes of your Silkies and let us know how they are doing over the next couple of months. Fingers are crossed for them!