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Photo: Getty images. Posed by models.

Only when sun sets on the Southern Chinese city of Mengzi, do the roller doors of the city's red light district rise. They reveal tiny lounge rooms bathed in pink light where young women put on stilettos and apply makeup. Strangely intimate, domestic scenes where a poster on the wall of a naked couple hints at what's for sale. In the hours before the men of Mengzi end their workday - and theirs' can begin - the sex workers gossip and do each other's hair.

The rags-to-(relative) riches tale of this 300,000 person city in Yunnan province is textbook modern China, and with prosperity sex follows. Add increased internal migration, where job prospects may put hundreds of miles between husbands and wives, and a gender imbalance that's left the country with a bride shortage, and China's sex trade – though technically illegal - is suddenly booming.

In Mengzi, the 30 or so brothels of Zhao Zhong Road represent just one, highly visible, portion of the city's sex industry. And on the sliding scale they - along with hotels, massage parlors and KTV joints – are considered high end. Their sex workers range in their teens to late 20s, happy to make fast money over slogging for low pay in a restaurant or factory. 

But further back in an older, shabbier part of the city is another class of sex workers. They work out of their homes, earn less and have little support on the occasion a customer is violent or leaves without paying. And with Mengzi just a few hours drive from the "Golden Triangle" of opium-production, some of these women do sex work to feed their crippling heroin addiction. In terms of social status they're just the gum on the bottom of the nation's shoe, but a priority for certain groups working to combat HIV. 

Overall, HIV is still a low-prevalence epidemic in China with just 0.058% of the population infected or 92,940 reported cases, according to the government's 2012 China AIDS Response Progress Report. Rather the disease is concentrated in "hotspots", with Yunnan accounting for the country's highest number of reported HIV cases. Where in the past injecting drug use and unsafe blood handling were the most common ways to be infected, cases from sexual transmission has leapt from 33.1% in 2006 to 76.3% in 2011 nationally.

30 year-old Ruan Shiyuan first discovered she'd contracted HIV in 2005, most likely due to unclean needles. With her single mother busy working as a brothel boss, growing up in Mengzi Ruan was often left to her own devices. She took up heroin at 17 and began working in the sex industry a couple of years after that. Nationally HIV is at a relatively low level among sex workers (0.3%), but the opposite is the case where sex workers are engaging in drug use.

Sweeping up scattered seed shells and a used condom off the floor, she asks if I mind she eat as we talk. Lunch is a piece of watermelon and a small bowl of plain noodles. She's painfully thin, and every morning takes 200 mg of heroin followed by a dose of methadone from the city's clinic in the afternoon. The evenings are for work where on average she'll see two customers and make 100RMB, half of which is spent on drugs. 

Ruan's customers don't know she's infected, but she insists on using a condom and is on medication, greatly reducing the chance of transmission. For Yin Li, project executor and CFO of Kangxin Home, these are important first steps. Each week Yin checks in with Ruan and other marginalised sex workers in the city. When they ask she helps them get tested for STDs or apply for the government's subsidized HIV treatment program. Occasionally she covers the 5RMB fee for their methadone dose.

Workers like Yin are an indispensable bridge between local health officials and sex workers, the latter often wary of authorities due to the illegality of their work. And yet community-based organizations such as Kangxin Home are in a period of change as many international aid programs pull out of China, and groups attempt to establish new co-operations with government bodies. Yin reports a positive working relationship with local officials in Mengzi.

But more generally the national AIDS response is still a 2.2 billion RMB a year work-in-progress. Like so many aspects of Chinese politics, strong national policy such as the 2003 'Four Free and One Care' initiative (offering free testing, medical treatment, counseling and education for orphans of AIDS patients), is weakened by patchy implementation as directives trickle through the country's five levels of government. More recent policy work is focusing on improving education programs.

Before her infection, 42-year-old Na Yan knew very little about HIV. Raised in a household of opium addicts, by 13 she too was hooked. Her mother, father, brothers and uncle have long passed away, but not before spending tumultuous lives selling drugs and doing time in labor camps. By 1995 heroin had replaced opium as the drug of choice, which Yin tells me is more potent, harmful and addictive.

But a few years ago Na began to replace some of her daily half a gram of heroin with methadone, and since last February completely quit drugs. Since then her health has improved, and she's put on weight, yet still cuts a fragile figure folded up on a small stool in the shadow of a cupboard, while Yin and I sit on her bed.

Both Na and her boyfriend of 10 years were diagnosed with HIV in 2008 but refused treatment because of costs related to clinical tests. Heroin had also turned their days short, investing in the future felt "meaningless", she says. In her boyfriend's final and painful six months in 2011, when he could no longer keep food down, she alone took care of him. Na now lives a lonely life on the second floor above a mahjong parlour. Her room is small but immaculately tidy 

I ask Na when she is happiest and she says, "At work, time passes quickly there." She recently began evening shifts collecting parking fees. It is menial work but a welcome change following a lifetime of selling drugs and then sex work since 2002 (in which she'd rarely use a condom). Still without medication Na frequently becomes sick - fevers and colds that she has left her body weak - but tells Yin she wants to register for HIV treatment. 

Spring Festival in China is traditionally the time to clean one's house and look to the New Year. And seeing Na, weak but not broken, I had to believe that it was possible for a heroin addict to escape prolonged suicide. Unlike Ruan who earlier that day had told me, "I have no future nor hopes for the future. I live day by day," Na's past Spring Festival marked the beginning of a new chapter. "I want to keep working hard at my job, get healthy and keep drugs and sex work out of my life," she says