At the height of the Sars crisis, Wei Wujun's phone was almost silent. 'No husband dared to risk going out to cheat on their wives,' says the Shanghai-based private eye and China's most famous mistress hunter. 'But today, with Sars off the radar screen, my phone is ringing non-stop again.' The outbreak didn't just have a dramatic impact on the mainland economy. It also had a devastating impact on mainland nightlife as most hostess clubs had to shut down during the 10 weeks that marked the height of the crisis, from April to early June. But with the World Health Organisation travel advisory on Beijing lifted, mainland and expat men are at it again: cheating on their wives. Mr Wei's business is seeing a revival. Frequently interviewed on television and known nationally as an er nai sha shou, or mistress assassin, the 48-year-old has helped more than 1,200 women track down their philandering husbands all over China. At 1.81 metres tall and 90kg, Mr Wei cuts a formidable figure. He usually wears sunglasses, even at night, and often wears a baseball cap that covers much of his face. A chain smoker, he is a quiet man who seldom boasts of his success as China's best-known private eye. 'You can't stop a married man from playing around, but when he takes a mistress and actually sets up a household, then he breaks the law,' says Mr Wei, who spent more than 10 years as a police officer in the People's Liberation Army in Shandong province and 10 years as a news reporter and TV producer before setting up his agency in 1993. 'Er nais [second wives] hurt the institution of marriage, and too many married men these days are taking them on.' Mainland mistresses aren't hurting only mainland wives. Just ask Laura, a 39-year-old who lives in the American Midwest. Laura, who asked that her last name not be used, knows about mainland concubines only too well - Mr Wei helped her track down her philandering husband and his mistress in Shanghai. For years husband Kevin had been a loving man. On Valentine's Day, he always gave her a dozen roses. On her birthday, there would be a romantic dinner out, away from the kids. At night, the couple always had quality time talking about what happened during the day. But all that changed two years ago, when Kevin began travelling to Shanghai for regular business trips. He became distant and cold. Then he didn't return phone calls until the next morning and began lying about money. 'Being in this situation as a wife is like having your head and your heart constantly arguing,' says Laura, who is 11 years younger than Kevin. 'I knew if this was anyone's else's husband, I'd believe another woman was involved, but because this was the man I married, had children with and thought I knew better than anyone in the world, I simply could not believe it. Any time I'd question his behaviour he said I was crazy and would vehemently deny any wrongdoing.' In July last year, Laura hired Mr Wei to snoop on Kevin during one of his trips to Shanghai. Laura's intuition was right: there was another woman, a 28-year-old from Shanghai. The detective caught Kevin and the woman on camera cuddling in a park, walking hand-in-hand in a shopping mall, and walking to an apartment he had rented for her. The evidence was presented in a divorce case Laura filed after Mr Wei tailed the pair for two days last July. She won more than half of Kevin's assets in the settlement a few months ago. Laura was a rare foreign client for Mr Wei, who says that more than 90 per cent of his customers are middle-age mainland women, usually angry wives of rich businessmen or senior provincial officials. He also serves a flourishing customer base of Taiwanese wives. On occasion, his clients are angry husbands who want him to track down their philandering wives. 'It's not because Chinese wives aren't satisfying their husband's needs,' says Mr Wei. 'China's spiritual vacuum is the problem. Before economic reform began, everyone was poor and relied on the richness of Maoist ideology. Deng Xiaoping overturned Maoism with economic reforms, but he lacked an ideology. What has come to fill the vacuum has been the worship of materialism - people in power and those with money are never satisfied. They need more power, more money and with it more sex.' Mr Wei may sound like a priest or a prude, but he is operating on the side of the law. Two years ago, the National People's Congress promulgated the New Marriage Law, which criminalises keeping an er nai. Although it does not carry a prison sentence, men caught keeping a mistress must compensate their spouses financially in a divorce settlement. Mr Wei's rates aren't cheap. He charges most clients 2,000 yuan to 3,000 yuan (HK$1,890-$2,830) a day, depending on the equipment needed to track down and spy on cheating husbands. For foreign clients like Laura, he charges US$1,000 a day. Despite the high fees, he runs a brisk business. 'The need for my services is tremendous,' he says. He employs more than a dozen full-time staff in Chengdu and Shanghai and operates an agency with a network of more than 200 independent detectives throughout the mainland. 'Many people have money and get rich. Once they're rich, they're no longer satisfied with one wife. Some have seven or eight 'wives',' Mr Wei says. Li Yinhe, a professor at the Chinese Academy of Social Sciences in Beijing, thinks Mr Wei is providing a much-needed service. 'Chinese society is regressing to before 1949, when it was acceptable for Chinese men to have concubines,' says Ms Li. 'Now, a wife whose husband has a mistress can sue for divorce and get a large chunk of his assets. That is why this new marriage law is so important.' The law came about as a result of heavy lobbying by Professor Li, who saw concubines becoming a national trend among businessmen over the past decade. Mr Wei says helping wives track down their cheating husbands isn't easy. Occasionally, he takes on mobsters. On a recent trip to Lanzhou, Gansu province, he had to flee from his hotel room to avoid six thugs that the mistress had hired to hunt him down. He was later able to follow the thugs to the woman, who he filmed with his client's husband. On another trip, he claims that he fought with 14 thugs hired by an angry husband who had grown tired of being tailed. 'I was a major in the PLA police corps and an expert in the martial art of qin na [a Chinese martial arts form similar to Japanese aikido],' Mr Wei recounts while taking drags on his cigarette. 'They followed me into an alley and tried to corner me. By the time I was finished with them, there were 14 of them lying down, and I walked away.' He admits he doesn't take on all clients. 'I never accept clients who are the wives of senior central government officials. You don't want to anger a senior official in Beijing. It's just too much trouble.' Mr Wei sticks to flourishing business in southern China, the mainland's prosperity belt. 'I plan to do this a few more years,' he says with a laugh, tossing a long mane of grey-streaked black hair that is usually hidden under his cap. 'And then I plan to open a detective school training other private eyes - other mistress assassins.' Laura says she hopes China can conquer its addiction to concubines. 'My advice to men is simple: don't hurt your families. When you have an itch or a mid-life crisis, don't hurt your wives and your children. Don't get an er nai. Get a convertible.'