Rocks, I have my degree in molecular biology and I have studied viruses a lot. Hypericum doesn't kill the virus. There are very few antivirals which actually get rid of a virus once it has reached a certain titer (that refers to the concentration of the viral organisms in the body). Birds are so very stoic that once we actually notice that there is an infectious agent attacking them, it is usually so rampant in their bodies that they've actually reached a tipping point in their infection. The hypericum is either supressing the virus enough that your birds body is able to heal somewhat or it is allowing her body to heal faster than the virus can cause damage.
I am excited to hear that you and other people are having such success with hypericum and I would not be of the opinion that you should cull this lucky chicken. When I was a kid, Mareks was a slow, painful death sentence. The fact that this herbal tincture can reverse these horrible symptoms and prolong their quality of life is wonderful to hear. I don't know enough about the virus which causes Mareks to say whether it will go into some kind of permanent remission or if you will need to keep her on the tincture for the rest of her life. But I do know that Mareks is a very unique virus in the way that it is passed from one chicken to another. I would assume that all of your affected birds contemporaries have already been exposed to mareks so putting her back with your flock shouldn't make a difference, but if you get or hatch any new birds, might I suggest that you make sure that they are vaccinated for mareks before exposing them to your existing flock.
