Campaigners say it is increasingly difficult to trace fearful prostitutes driven underground. Xiao Xiao wiped tears from her reddened eyes. She starts crying whenever she thinks about her days as a prostitute in a nightclub.

An employee with the local center for disease control tells an HIV-infected patient (left) how to take her medicine on Nov 25 in Yuxi's Hongta district, Yunnan province. Hongta's anti-AIDS workers have spent eight years trying to reach out to sex workers in an effort to educate them in the benefits of safe sex.

Sitting in a quiet coffee shop in Yuxi, Yunnan province, the petite, pale-skinned 30-year-old had agreed to talk about her struggle with HIV but became upset when the subject turned to her "disappearance" in 2002.

That year, she said, all employees at the club were given blood tests.

"I didn't know it was a HIV test," she said, although not long after getting the results, she left her job.

For three years, officials with the city's center for disease control (CDC) were unable to trace Xiao (not her real name).

She reappeared when she was hospitalized with a life-threatening fever.

"We were trying to contact you (all that time) but we couldn't find you," Li Lianxue, a doctor with the Hongta district CDC, said to Xiao as they chatted about the past over a coffee last week.

"I had no idea", was her only reply.

As China marks World AIDS Day on Wednesday, anti-AIDS campaigners say keeping track of HIV-infected sex workers is one of their greatest challenges - not least because of police crackdowns.

"Crackdowns do little for disease control, they just drive prostitutes further underground, which undermines efforts to reach them and deliver intervention education," said Jing Jun, director of Tsinghua University's social policy institute.

Following the closure of an infamous nightclub in Beijing in June this year, authorities across China, including in Yuxi, launched fresh campaigns against the sex industry.

"We are determined to put an end to ... illegal activities in the entertainment places," said Qian Jin, deputy director of the security corps under Beijing's public security bureau.

However, health professionals in Hongta, who have spent eight years raising awareness of HIV and AIDS, fear plans to shut the 100 or so "entertainment venues" they regularly visit will mean losing all contact with the sex workers employed there, including those infected.

Of the 614 women given health exams in the district this year, only one tested positive for the virus. She has already disappeared from the CDC's radar, admitted Li Lianxue.

"This year, we've given out more than 100,000 condoms at these venues but it's far from enough," said Ma Yi, deputy director of Hongta CDC, who said that a prostitute at a "hair salon" can use up to eight condoms a day.

Flipping through the monthly free condom distribution records, Li Jinlin, the CDC doctor in charge of contacting venues, said more than 70 "hair salons" were closed by police last month, meaning they now only have records for sex workers at 21 nightclubs.

"Experience has shown that cracking down on sex work and driving it further underground is likely to make sex workers reluctant to accept services and make it difficult for health workers to gain access to them," said Mark Stirling, China coordinator for UNAIDS. "Interventions that respect the rights of sex workers can be highly effective."

As prostitution is illegal in China, there is no official number for how many people are involved in the business, although experts estimate the figure is roughly 3 million nationwide.