If she had symptoms when you got her, chances are she brought an avian virus into your flock. These are contagious. It's important to know what virus, if it's a virus, so you can manage it properly.
I have an avian virus in my flock, most likely arriving with the first adult chickens I began my flock with from a friend's flock after she died in a highway accident. Every chicken and baby chick since has been exposed and now carry it. I am aware of it and practice strict bio-security to keep it in my flock and not export it by wearing different foot wear when I am tending to them than I wear when going out to the store.