How to Treat? Very sick chick...

Hey, all. I posted yesterday in a different thread about a sick EE chick of mine. We noticed yesterday that she didn't go into the run with her sisters and when I looked in, she was sitting all puffed out and doing this thing where she extends her neck and opens her beak wide. She looked worse when I came home from work. We have her separated. She looks better and worse this morning- more alert (not so sleepy/lethargic) but her toes are now curled and she can't stand without falling.

I saw her eat last night but have not seen her drink. I want to go get anything I can to help her, but not sure what to get without knowing what's wrong? (I read for hours last night and just couldn't perfectly fit all her symptoms to anything.)

I have apple cider vinegar that I can add to her water. What else would you recommend for a very sick chick? Antibiotics? Medicated feed? I've heard of electrolyte packets and all kinds of things, but not sure what's best! And I don't want to overwhelm her system. Any advice??

You can see a video from yesterday morning (she looks considerably worse today) & more info on my other thread:
https://www.backyardchickens.com/t/1099322/opinions-on-this-video-sick

My initial thoughts are Mareks, which causes internal tumors which leads to paralysis.

That's what I'm fearing, too. She looks a little better this morning, putting a little weight through her legs and she drank on her own, which she didn't do yesterday, but she still doesn't look great.

Was she vaccinated for Mareks disease?

I'm not sure. We got our first batch of chicks from a hatchery and had them vaccinated. The new chicks we got from a farm and I guess we stupidly assumed that they were. I am so mad at myself.

It could also be botulism, which can cause similar symptoms. Its mostly from consuming moldy feed. It can sometimes improve.