Marek's is contagious, but if that is what it is then all your birds have already been exposed. It is very easy to jump to the 'worst case' assumption, but that is not always the correct thing. If she does not improve or continues to worsen, and you lose her, it would be a good idea to consider having her necropsied, just in case. Better to know for sure than to assume wrong. You can also give her some vitamin E daily with a bit of tuna or egg for selenium (that helps with the uptake of E), just to cover all the vitamin bases.
What do you feed, including treats? And is she low in the pecking order? Low status birds can be kept from feed, and some whole grain feeds allow them to pick and choose bits which can set up deficiencies.