That, for the Centre for Health Protection, represented an "unusual increase" in human immunodeficiency viruses, which causes AIDS, among women engaged in the sex trade.

One of the five developed an acute condition while the others were asymptomatic. All of them have since received medical care and are in stable condition.

The five were mainly active in Yau Tsim Mong and Sham Shui Po, with no known common source of infection.

Wong Ka-hing, head of the center's public health services branch said the five cases were recorded between August and last month. He added that the situation was alarming and urged sex workers to stay vigilant.

Wong also stressed the importance of regular testing as all five had tested negative for HIV in the three to 19 months prior to their positive test result.

The center has worked to enhance health education and preventive interventions among sex workers and their clients, Wong said, adding it had also reinforced testing and counseling services.

He cited the Department of Health's social hygiene services as having done good work in promoting awareness of HIV. Sex workers and their clients should attend its social hygiene clinics for screening tests, Wong added.

"We would like to appeal to people who have engaged in unsafe sex to undergo an HIV antibody test," he said. "They can call the department's hotline at 2780-2211 for a free, anonymous and confidential test."

With unsafe sex being the main cause of HIV infections, Wong urged the public to play it safe.

According to a fact sheet published by the Hong Kong Advisory Council last June, the city logged an average of between 60 and 70 HIV cases among heterosexual women every year for the last decade. The figures skew slightly higher for heterosexual males, with an average of around 10 more cases per year.