But Nadja Benaissa didn't tell the men she slept with. And in Germany – where she was a member of the country's most successful girl band – that is a crime. Hannah Booth on a very public downfall

Nadja Benaissa's story has all the improbable drama of a soap opera: sex, drugs, teenage pregnancy, secrets and blackmail. Behind the headlines, though, is a more familiar story: the daughter of an African immigrant caught between her strict upbringing and a desire for freedom.

Until recently, Benaissa, 28, was a member of Germany's most successful girl band, No Angels – the first pop group in that country formed on a TV talent show. For nearly 10 years, her days, like those of her four bandmates, consisted of chart-topping singles, million-selling albums, sell-out tours, public appearances and a lot of make-up. But in April 2009 she made headlines for a different reason. She was arrested on suspicion of having unprotected sex with three men while knowingly carrying the HIV virus, and for subsequently infecting one of them.

The tabloid press vilified her; the broadsheets condemned the manner of her arrest, calling it a "witch-hunt". A highly charged trial took place last August, during which intimate details of her sex life were revealed to the world.

On a bright morning in west Berlin, we sit down to talk. Benaissa is warm and speaks thoughtfully in soft German. She was born in Frankfurt in 1982 to a Moroccan Muslim father and a German-Serbian mother. She was a sporty, musical child, and close to her brother and parents, particularly her father, Muhamed. But their relationship changed when she hit puberty early, aged 11. "My father was overwhelmed by it," she says. "I was always a daddy's girl, so when I became a young woman he didn't know how to deal with it – in Morocco, that's when men distance themselves from the women in their family."

As a result, he became stricter. Sex was a taboo subject at home – if characters kissed on television, her father would switch it off. "Anything I liked, he hated. He thought I was off meeting boys when I wasn't. It made me more defiant, and I started doing things just to spite him."

She got in with the wrong crowd, smoked, drank alcohol and lost her virginity at 14. She smoked marijuana, moving on to ecstasy, cocaine and then crack, which she smoked without realising what it was. "Crack felt different," Benaissa says. "I noticed how badly I wanted more, even after one time." She started spending longer periods away from home, sleeping around, stealing and living with dealers who supplied her with crack.

The drug quickly took its toll, and she became emaciated, her body riddled with sores. "I didn't eat, drink or sleep. When I looked in the mirror, I didn't recognise myself." She was 16.

Around this time, Benaissa found out she was pregnant. Doctors told her she would be better off having an abortion, but she saw it as a chance to straighten herself out: "I had one motivation, to bring a healthy baby into the world."

A few weeks later, back in hospital for a routine check-up, she found out she was HIV positive. "I was terrified. The doctors actually told me I had Aids and had about eight years to live." This was the late 90s, when HIV was widely misunderstood and still seen as a death sentence. "They were old-fashioned, and angry with me."

A more sympathetic doctor explained she had HIV, not Aids; that it was highly unlikely her unborn child would contract the virus; and that if she stayed in good health, she would live a long life and, more significantly, there was only a small chance she could pass the virus on.

From whom did she contract HIV?

"I will never know for sure, but I can imagine."

Did she tell him?

"No, I was only thinking about me. We didn't have a relationship, didn't know each other at all. I wouldn't have been able to talk to him about it."

But if she had any suspicions, surely she should have told him, so he didn't go on to infect someone else? Benaissa seems confused by the question. "Yes, he could have, but so could many other people who didn't realise they had [the virus]. I don't think he did it intentionally. I have a very rare strain of HIV – only half a dozen people in Germany have it – so I don't think he's going around infecting women. I don't even think he's in Germany any more."

Benaissa's daughter, Leila, was born in 1999, without HIV. Life took on a degree of normality – she adored her daughter, took school classes a few days a week, and started singing and playing the piano again.

The following summer, she saw an advert for Popstars, a TV talent show looking to put together an all-girl band. Benaissa applied and sailed through the selection process. During the final round, when she suspected she was a favourite, she phoned the producers. "I said, 'I can't do it. I don't want to be in this band.' My daughter wasn't even one year old."

But the producers were persuasive. And when she won, it was an opportunity she couldn't turn down. "My mother said to me: 'Imagine how you'd feel if the group became famous and you weren't a part of it.' And she was right."

No Angels' catchy debut single, Daylight In Your Eyes, was released in February 2001 and went to the top of the charts. Their first album, Elle'Ments, did the same. They sold 5m albums between 2000 and 2003, had four number one hits, and their every move was chronicled by teen magazines, tabloids and television.

Benaissa burned out within months. "I was completely unprepared, overwhelmed with the work and the fame. I enjoyed being with the girls, singing, recording, but it was more about keeping the media interested – there was always a camera in your face. And I had no time for my daughter."

Naturally, there were men – some flings, some longer relationships. It was easier to tell more serious partners of her HIV, she says. "We would get to know each other more slowly, and I would explain my situation. And then he would decide whether or not to stay."

But with shorter affairs, it was harder to bring the subject up. She was a celebrity. Only a handful of trusted friends and family knew about her HIV. As a result, Benaissa had unprotected sex with three different men on half a dozen occasions.

Why didn't she insist on using condoms, whether or not she revealed her HIV status? "It's hard to say," she says. "I don't really understand it myself. It had to do with the moment: sex is passionate, spontaneous. It was a moment of weakness, a mistake."

One of these short relationships was with an art dealer six years older than her, a friend of her aunt. They'd known each other a short while before they slept together – no more than a dozen times, sometimes with, sometimes without protection. She didn't tell him she was HIV positive.

"I wanted to but couldn't," she says. "We'd drunk a lot of alcohol, it all happened very quickly. Afterwards, I was too scared to say anything. It was a vicious circle of keeping quiet, fear and denial. I talked myself into believing nothing would happen. Then I discovered a few years later that something did happen." Her voice tails off. "And it was too late."

Three years later, in 2007, Benaissa heard that this man was infected with HIV, presumably by her. What was her reaction? "Shock. Fear. Panic. Rumours started online, so my first thought was how to avoid a huge scandal – we were in the middle of our comeback tour and I felt responsibility towards the band."

She did feel guilty. "I had to take responsibility for that. Up until that point, I'd come to terms with this disease. If I did tell people, they were shocked, but also compassionate. That changed when I infected this man. I became a culprit.

"The responsibility lay clearly with me. I should have insisted on protection, as I knew I was HIV positive. But, generally speaking, I think individuals should take responsibility to protect themselves, because you never know who has HIV – many people don't know themselves."

As the months passed, the rumours continued. The German tabloids, by now on to the story, tried to blackmail Benaissa into declaring her status. In April 2009, after waiting nearly two years, the art dealer went to the police. In Germany, failure to disclose HIV-positive status before having unprotected sex carries a prison sentence of up to 10 years.

Benaissa was arrested in a Frankfurt nightclub just as she was about to appear on stage; she was handcuffed by plainclothes policemen and then held in custody for 10 days. The media reported her arrest, but didn't initially print the reason for it. That was made public when, in a much-criticised move, the prosecutor issued a press statement. Germany usually protects the privacy of its suspects. The prosecutor's act broke federal law, said her lawyer, and made the assumption his client was guilty.

Benaissa's trial took place in August 2010, when she faced the man she was accused of infecting for the first time since they had slept together. Giving evidence, he shook with emotion. "I didn't know if I would be dead from one day to the next," he said. "You have created a lot of misery in the world."

"It was hard, he was obviously very angry," Benaissa says now. "I saw how he hadn't come to terms with it, and that his life had really gone off the rails. That upset me a lot."

Before the trial, she tried to talk to him, but he didn't want to. "I will keep trying, but if he doesn't want to see me, then I have to accept that. I apologised to him during the trial, but you can't just say sorry and make everything right."

An HIV expert told the court that Benaissa was almost certainly the source of her former lover's infection, because they both had a rare strain of the virus. Other experts disagree: "At that distance [more than 10 years], there is almost no chance of proving if person A has passed a virus on to person B," says Lisa Power of the Terrence Higgins Trust.

Benaissa told the court: "In those days, I was careless. I'm sorry from the bottom of my heart. I never wanted this to happen to any of my partners. The fact is, I made a great mistake." Her contrition was a factor in her sentencing – she was found guilty of grievous bodily harm and attempted bodily harm, and received a suspended sentence.

The trial has sparked debate in Germany about the merits of criminalising those who knowingly transmit HIV, since the disease is no longer a death sentence and legislation fosters the stigmatisation of those who carry it (there is no equivalent law in the UK) [see footnote]. "Prosecutions like this don't help," Power says. "They make it harder for people to come forward and ask for help. People with HIV should make every effort to avoid passing it on. But some struggle with disclosure for a number of reasons, and they need support to manage safer sex."

If anything positive has come from her trial, it's that these issues are out in the open, as is her status. She is no longer at the mercy of would-be blackmailers, and has recently taken on Aids awareness work.

Her relationship with her father is also better now. "He has become a different person," she says. "We can now talk about sex, which is a huge deal."

As for the band that made her name, she announced in October that she had quit for good. She had become bigger than the group, she says. "The scandal, the trial – they were overwhelmed by it." She appears happy with the decision. "I always wanted to be a singer, and will continue to be, but not this whole pop star packet. This HIV thing will always be there, but I think acceptance of it will grow with time. They'll be fine without me."

• This footnote was appended, and the following correction published, on 23 March 2011: We state that "there is no equivalent law in the UK". To clarify: there is no specific statutory offence, but charges of grievous bodily harm have been brought against individuals in similar circumstances under the Offences against the Person Act.