You will probably never know how the chick contracted the disease - it can come in on shoes, car tires, wind - the possibilities are endless. You could have a carrier in your flock that is shedding the virus without your knowledge. Vaccinated birds do not infect other birds - the vaccine is not the infective wild strain of mareks. I would advise vaccinating everything on your place - while your best chance of preventing the virus is by vaccinating & immunity prior to exposure, vaccination of adults or exposed birds will not do them any harm and may provide some resistance. Going forward, try to get vaccinated birds or keep new birds in isolation and vaccinate them prior to exposure to your flock. Keep in mind that if this youngster has mareks, she is shedding the virus and is really a "typhoid mary" for the rest of your birds. For this reason, I no longer attempt to treat birds that appear to have mareks, but euthanize them immediately and disinfect like crazy afterwards. If you have a place to have a necropsy done, you could find out for sure if it is mareks and take the precautions necessary to protect the rest of your flock.