Help! Chicken's legs wobbly, getting worse!

[piper123]

I have a five week old chicken that seems to be having issues walking without wobbling. Her legs shake when she walks and she keeps falling onto her side or butt. She has a super long stride and picks her feet up much higher than normal to walk. It just started this morning and I've never seen her have an issue before. She has no other symptoms, is eating and attempting to run and scratch with the others. Her legs don't seem swollen or anything either. Does anyone have an idea of what this could be?

Update: I gave her an Epsom salt soak and put some apple cider vinegar in their water in case something is inflamed, but I really don't think anything is. She's getting worse and won't attempt to stand or walk anymore. Her legs just slide out to the side of her and she sits down. What could progress so quickly?

 

[piper123]

Side note: we hatched these chicks ourselves so they were not vaccinated against marek's disease, but they've been kept isolated from our older birds who of course have no symptoms. I don't know how prevalent or contagious the disease can be.

 

[DolceVita]

@Eggcessive