The virus, known as the Human T-cell Leukaemia Lymphoma Virus-I (HTLV-I), can be contracted in similar ways to HIV. However, HTLV-I does not lead to AIDS. There is no vaccine.

The rare virus has previously been found mostly in prostitutes and drug addicts in Taiwan, a popular tourist spot for Hongkong people, and in southwest Japan.

A special group comprising experts on lymphoma from the Queen Mary and Queen Elizabeth hospitals has been set up to investigate the case.

Associate dean at the Hongkong University's faculty of medicine, Dr Raymond Liang Hin-suen, said yesterday the Department of Health had been notified of the detection of the virus.

Hongkong doctors have been researching it for more than 10 years.

Asked if it was possible for the virus to spread in Hongkong, Dr Liang said: ''There is a possibility because people visit brothels in Taiwan.

''Even though the risk of being infected with this virus is lower than with HIV, there is still a possibility.'' Dr Liang said up to 20 per cent of the population in certain parts of Japan were carriers.

The woman patient in Hongkong told doctors that she had not indulged in high-risk behaviour.

Dr Liang said: ''We are doing genetic typing to find out the source.

''No such case had come to our attention before.'' Dr Liang said it would take months to get the results of the study.

He said HTLV-I was less infectious than HIV, because it reproduced slowly and people could be infected only after prolonged contact.

An average of 40 per cent of lymphoma patients could be cured, but there was a high relapse rate. Experience in Japan showed that 80 to 90 per cent of patients relapsed.

Dr Liang said the most important step was to find out whether the virus originated in Hongkong.

Queen Mary Hospital looks after a third of lymphoma patients, while another third are treated at Queen Elizabeth and the rest at other institutions.

Most lymphoma patients are between 20 and 30 years old. Hongkong has 400 to 500 new patients every year.