A two-month-old girl is in a hospital isolation ward after being confirmed with H9N2 bird flu yesterday - the day live chicken sales resumed after a 21-day ban sparked by an outbreak of the much more serious H5N1 strain among birds at a Yuen Long farm. The baby, who is believed to have contracted the usually mild disease in Shenzhen, where she lives with her parents and grandmother, is in stable condition in Tuen Mun Hospital. Her parents and grandmother are under a week's medical surveillance at the family's Hong Kong home while tests continue to see the virus has mutated into a potentially more dangerous form. Staff at the hospital have been put on preventive doses of the antiviral drug Tamiflu. The case - the fifth such infection in Hong Kong since 1999 - worries the Hospital Authority because it comes in a peak period for flu infections. The baby, who was born in Tuen Mun Hospital, was admitted on December 22 with an upper respiratory infection, though she did not have a fever. Tests showed she had influenza A. She was discharged a day later and returned to Shenzhen. She was readmitted on Monday after a doctor in Shenzhen found she had poor appetite and a low blood count, unrelated to the flu - from which she has recovered. The girl is now being treated for suspected leukaemia. Yesterday the results of further tests showed she was suffering from the H9N2 flu strain. She is believed to have caught it in Shenzhen from passing close to a wet market or area containing live poultry. Thomas Tsang Ho-fai, controller of the Centre for Health Protection in Hong Kong, explained why there was a time lag in confirming that the virus was H9N2. 'The amount of virus present initially may be very small and so it needs to grow in cells. When it grows to a certain amount then we can detect it and confirm it to be H9.' The Hospital Authority said all public hospitals had been asked to stay vigilant in the surveillance of flu-like cases. The 'serious response level', raised on December 9 because of the outbreak of H5N1 in Yuen Long, remains in force. The co-ordinator of the World Health Organisation's global influenza programme, Keiji Fukuda, said the case was a reminder that many viruses had the potential to cause an influenza pandemic.