How long has she been sick? Where are you? (what State or are you in the US?) Have you dewormed your flock? Have you checked for external parasites? (lice/ mites) Does she have any respiratory symptoms? Swollen eyes? What are her droppings like? Is she underweight? Can she walk OK and use her wings?
You need to separate her from the flock and put her in a hospital cage.
If you can answer the questions above, we can help you better.
