Just got a call from Penn State. They confirmed that Candy died from Marek. Her has microscale size of tumors in most her organs, which cannot be found by naked eyes.

She had minor coccidiosis too, which is not the cause of her death, but Doctor Dunn suggests that I don't need to treat the other chickens. She also said many chickens have high resistance to Marek, but Silkie is not. I explained to her that Candy is not a Silkie, but Ameraucana. Anyway, so I will have to vaccinate my chicks in the future for safe.