NG Wing-tun played happily in the paediatric isolation ward at Queen Mary Hospital yesterday, unaware that a simple infection could kill her within hours. Two-year-old ''Tung-tung'', as she is affectionately known, has aplastic anaemia. Her bone marrow has stopped working and her body cannot produce enough white blood cells to stop bleeding. She has just survived ''a very serious bout'' of bleeding into her brain. She cannot have normal surgery because her blood cannot clot and the treatment she is getting is described as ''maximum support''. A bone marrow transplant is her only hope. As she played with her stuffed toys and chatted happily, her parents spoke of the crushing disappointment they felt, knowing four possible donors had been identified, but none could be reached. Her father, Ng Yiu-king, said: ''We had almost lost all hope when the news came through from the Hong Kong Marrow Match Foundation registry that four possible donors had been identified. Our worry lessened, knowing our baby had a chance. ''Then we were told the registry was unable to reach any of them. I understand they were among the thousands who gave blood samples to help 'Little Gordon' from Canada two years ago. Maybe they have lost their enthusiasm. ''But, I beg them, and anyone else who came forward then, to please consider helping my daughter and other children like her.'' Tung-tung's physician, Dr Anselm Lee Chi-wai, said the child's condition had been diagnosed three months ago and she had less than a 10 per cent chance of living. ''Ninety per cent of these patients die within the first year of diagnosis because infection is their number one enemy. Any ordinary bacteria could get into her blood stream and poison her. ''If that were to happen, she could be dead within hours. This child's case is very, very serious. We need a donor urgently,'' he said. Even before he explained, the ravages of the illness were plainly visible. Tung-tung's left eye was bloodshot and the bruising around the forehead and under the eye indicated the internal haemorrhaging. She is chubby from the massive doses of steroids she has been given. Until a donor can be found, she is being treated with ATG, an antibody produced in horses. It is an immunal suppressive treatment, and the best medicine can do - without a transplant. But her response to it has been just 30 per cent. Her parents are so desperate they have asked the hospital to consider any form of experimental treatment to save their daughter. Mother Hon Sau-ha never suspected her daughter suffered from anything so serious. Until Tung-tung started coming up in unexplained bruises three months ago, she had been a normal toddler. ''The pain in my heart is suffocating. I know every day that my child can leave me at any time. She just turned two on October 31. I want my daughter to live. She deserves a life.'' Operation Santa Claus, conducted by RTHK Radio 3 and the South China Morning Post , aims to help children like Tung-tung by paying for the tests needed to register new donors. Increasing the donor base would increase the patients' chances of finding matches for bone marrow transplants. The other beneficiaries this year are the Paediatric Bone Marrow Transplant Unit at the Prince of Wales Hospital and a halfway house for recovering transplant patients.