They can die from Mareks. If it's only her foot though, I wouldn't say Mareks but I'm not a professional. Does she fall over on her side?
There is a cure for Mareks. If that's what she has, get some... hang on here. I'm having trouble remembering the name. You can get it at a health food store, it's made from St. Johns Wort, and they make it into little tiny pills. Talk to the people at the health food store.
If you find some of it, DO NOT touch the pill with your hands. First, put some water into a teaspoon. The lid of the pill container turns to dispense a pill when needed. Make sure the pill drops into the water and then let it dissolve. I think this takes about an hour. Then you take an eyedropper and suck up the fluid in your teaspoon. You'll need help with this so have an extra pair of hands available... You need to drip 5 drops of this liquid into the crevice on the roof of your bird's mouth, make sure it goes into the crack. A week of this is all she should need.
I haven't had Mareks in my flock for quite some time but I used this on one male that I had that had recurring leg problems and it really did the job for him. I usually gave him 8-10 drops but he was a big bird and I wasn't positive I got them all in the crevice.
Let us know how it all works out.
Just a tip for those who want to know... regarding Mareks.
Turkeys get a form of Mareks that chickens can catch. The turkey form is much milder than the chicken form. If you have turkeys available to run with your chickens, you'll probably never have the fatal chicken Mareks. A chicken catches it and it is so mild that you can't even tell they got it but it makes them immune to the chicken form. If you're not comfortable with running turkeys with your chickens (some people are) you can probably get turkey droppings from someone else who has them. Just spread it around in your chicken yard. I know the bio-security people are probably pulling their hair out right now but it works and as long as the turkeys you get this from are healthy, there shouldn't be any issues. I did it and my birds were all fine and guess what else... I haven't had Mareks in my flock for 5 years. I've been raising my birds for 6 years and the first year I lost over half of my babies to Mareks so I'm convinced.