"There's no shower in the wild. Each girl will serve dozens of men every night, so most of them would have contracted some form of sexually transmitted disease on their outer vaginal region, even if they use the condoms we hand out."

Because of its large population, China has the most sex workers in the world. But the country's sex industry operates entirely underground, because the Communist Party-led government bans any kind of sex trade. Experts say the ban - coupled with institutionalised corruption and rampant abuse by police with the power to order detention without trial - makes the working environment of those in the trade much tougher.

Professor Pan Suiming , head of Renmin University's Institute of Sexuality and Gender, puts the number of China's sex workers in 2010 at up to two million. The World Health Organisation and a May 2012 report by Human Rights Watch put the number at between four and six million. Other estimates go as high as 20 million, claiming that sex workers contribute as much as 6 per cent of China's gross domestic product.

The nation's underground sex industry comprises a wide variety of workers, from the traditional working girls commonly spotted in salons, saunas and nightclubs to high-end gigolos for women, male prostitutes for gay men, and transgendered sex workers.

According to Yang, a popular sex worker in Shanghai charges about 2,000 yuan for a two-hour session, catering to different fetishes. Some trained nurses also join the trade, performing piercings or circumcisions for their clients. Another significant part of the trade is web-based, with girls stripping or men having live sex in front of web cameras.

"Japan might seem to have a more developed sex industry, with a wide spectrum of adult video genres. But the sex industry in China is just as diverse," Yang says. "Just because Chinese law prohibits the production of adult videos doesn't mean that people who want them don't know where to find them."

Ken, 29, a male prostitute who services men, has been in the trade for five years now. The university graduate charges 400 yuan per session and 600 yuan to spend the night.

"Money boys usually operate from rental apartments, each with three to four bedrooms and managed by a male mamasan [or pimp]," he says. "Some work in health clubs, providing massage services and more. There are about 10 such clubs in Shanghai."

Many also operate individually, placing ads online and meeting clients in hotels, Ken says. "The apartments are always raided by undercover police. Once arrested, violence to extract a confession is not uncommon."

Being robbed by clients is another occupational hazard. "I was in the shower once, and the client brought in two men to rob me," Ken says. "I knew better - I had on me only a cheap phone and enough cash for a taxi home - but they beat me up."

The young man's family does not know of his sexual orientation or what he does for a living.

"I had a boyfriend two years ago; we lasted for six months. I'm ready for love, but I won't give up my life unless I'm driven out of the trade because of age," he says.

"I've seen many money boys marry women back home because of family pressure, but continuing their work in the big cities," he adds. "But that's not something I want. I'm happy where I am."

Guo Ziyang , executive director of Beijing Zouyou Information and Consulting Centre, says his NGO has offered sexual health advice and counselling to almost 4,000 gay and transgendered sex workers since 2007.

"No one knows how many sex workers there are, as they move quickly from one big city to another. The market is huge, so the internet is a popular place for them to look for clients," he says, adding that it costs 600 to 800 yuan to engage a sex worker in Beijing and 400 yuan in second-tier cities like Chongqing .

In the face of one of the nation's toughest crackdowns against sex workers in recent months, many saunas and nightclubs have been forced shut. Prostitutes and pimps who used to thrive in Dongguan , commonly known as China's sex capital, have disappeared overnight since the crackdown began there on February 10 before extending nationwide.

"The crackdown is a political battle to eliminate enemies in the name of eradicating the sex trade," Guo says.

"We all know such campaigns will never be successful. They've been carried out before, usually ahead of important events such as the Olympic Games and annual party meetings," he adds.

Xiang Xiang, 23, is one of the few female sex workers to agree to an interview during the police crackdown. She joined the trade at the age of 17 after realising that her years of hard factory labour in Guangdong would not earn her enough to pay her younger brother's tuition fees back in rural Hunan province.

"Factory work was a waste of time. I couldn't get enough money to send home," says Xiang Xiang, who quit school at 14. "I worked in a nightclub and was assigned with the number 18. I thought I was lucky to get such a good number; I'd get 300 yuan each time my number was called. I'd drink and sing with the customers and serve them food. If they liked me enough, they would take me out to spend the night for 1,000 yuan.

"I don't really remember what my first time was like, as I had passed out from drinking too much. I woke up next to a naked man in his late 40s. He got dressed, gave me the money and told me to sleep in if I wanted to. That was the first I realised how some women could earn so much money. I don't think about whether it's dirty money. I know only that just by sleeping with men, good money will come.

"Most of the time, I would be drunk, so sex has mostly been a blur for me. But once, I felt degraded after being forced to have sex with three men in the same room. I got 3,000 yuan afterwards, but I locked myself up for a week after that. That was the first time I thought about leaving the trade, finding a man and getting married. But it's impossible.

"Once, I fell for a client, a married man. In the end, it was just free sex for him for months until the day I got dumped. I don't believe in love any more."

Xiang Xiang says she makes a good living, earning 5,000 to 10,000 yuan a month. But she gets no business at all during police raids. "I can't go back to factories. Without any skills or experience, I'm a reject," she says.

Yet she believes she is one of the luckier sex workers in the country, as all the clubs in Zhongshan's Shiqi town where she works are protected by police for a monthly fee. "Our mamasans will just tell us to take a few days off during raids, it's all taken care of," she says.

Things are worse for the older sex workers who make their living serving migrant workers at construction sites, on the streets or in low-end brothels. An arrest can see them fined up to 10,000 yuan and detained for at least six months, Xiang Xiang says.

"There's no fixed penalty for the offence," says sex coach Yang. "It all depends on the mood of the policeman who arrests you. Sometimes, they might even let you go for free."

Professor Peng Xiaohui , of Central China Normal University and vice-secretary of the World Association of Chinese Sexologists, says the sex trade is the same everywhere.

"China's sex industry is at its peak right now, judging from the official response to the trade. But the crackdown is aimed at political opponents in the name of morality, to win public support."

He said sex work was an honest trade, with workers selling their bodies, compared to corrupt officials who cheated the state of public money.

"China's sex industry is characterised by institutionalised corruption. Sex workers are exploited not only by triads who control them, but also by corrupt police and government officials who make their lives even tougher."

He cites Sweden's sex industry, which was at its peak in the 1960s. "But as quality of life improved in Sweden, with fairer wealth distribution and a better justice and welfare system, the sex trade became less alluring."

To eradicate China's sex industry, the country must first reform, allowing political participation and the fruits of economic development to be shared across the nation rather than being in the hands of just a privileged few, Peng said.

"Most sex workers come from rural areas. They have no skills and cannot get jobs that will pay well enough, so they fall easily into the sex trade," he says. "If our welfare system is sound, the number of sex workers will be drastically reduced."