She reached out her hand in greeting. "Mei Ling," she introduced herself in a soft voice (her real name has been withheld). At first glance, the 37-year-old seems very normal. She has a sweet appearance, clear skin and is well presented.

But it is not long before you sense there is something different about her. She smiles, but almost without emotion. Her thoughts seem to have wandered off to a place far away. She has a flat, empty gaze, but o