Anyone following her updates online could see that Mandy Wilson had been having a terrible few years. She was diagnosed with leukaemia at 37, shortly after her husband abandoned her to bring up their five-year-old daughter and baby son on her own. Chemotherapy damaged her immune system, liver and heart so badly she eventually had a stroke and went into a coma. She spent weeks recovering in intensive care where nurses treated her roughly, leaving her covered in bruises.

Mandy was frightened and vulnerable, but she wasn't alone. As she suffered at home in Australia, women offered their support throughout America, Britain, New Zealand and Canada. She'd been posting on a website called Connected Moms, a paid online community for mothers, and its members were following every detail of her progress – through updates posted by Mandy herself, and also by Gemma, Sophie, Pete and Janet, Mandy's real-life friends, who'd pass on news whenever she was too weak. The virtual community rallied round through three painful years of surgeries, seizures and life-threatening infections. Until March this year, when one of them discovered Mandy wasn't sick at all. Gemma, Sophie, Pete and Janet had never existed. Mandy had made up the whole story.

Mandy is one of a growing number of people who pretend to suffer illness and trauma to get sympathy from online support groups. Think of Tyler Durden and Marla Singer in Fight Club, only these support groups are virtual, and the people deceived are real. From cancer forums to anorexia websites, LiveJournal to Mumsnet, trusting communities are falling victim to a new kind of online fraud, one in which people are scammed out of their time and emotion instead of their money. The fakers have nothing to gain from their lies – except attention.

These aren't just people with a sick sense of humour. Jokers want a quicker payoff than this kind of hoax could ever provide. It requires months of sophisticated research to develop and sustain a convincing story, as well as a team of fictitious personas to back up the web of deceit. Psychiatrists say the lengths to which people like Mandy are prepared to go mean their behaviour is pathological, a disorder rather than simply an act of spite. The irony is these people might actually be classed as ill – just not in the way they claim to be.

Some psychiatrists have started using the term Münchausen by internet (MBI) to describe this behaviour. Whereas Münchausen syndrome requires physically acting out symptoms to get attention from doctors, online scammers just have to be able to describe them convincingly. There's a potentially limitless audience of sympathetic ears, and success can be quantified by the number of concerned emails and message board posts generated by your lies. Some even go so far as to fake their own deaths, reading their own obituaries and observing the torrent of grief from the comfort of their living room. If they are rumbled – and they rarely are, conclusively – they just go to another support group, and to a fresh batch of trusting victims. The people they've fooled rarely find it so easy to move on.

No one suffered more from Mandy's lies than Dawn Mitchell, the 42-year-old Canadian who eventually exposed her. She became aware of Mandy's plight in 2007, when other mothers on the site signed off their posts urging for prayers and positive thoughts for their critically ill Australian member. "When you've got young children of your own, hearing about another mother who might leave her children motherless makes you more emotional."

She began providing the round-the-clock emotional support that Mandy craved. "I could spend an hour with her in the morning, a couple of hours in the afternoon, then I'd be up at night after the kids went to bed, sometimes until one o'clock. Instead of having a glass of wine with my husband in the evening, I was on the computer listening to Mandy talk about her latest infection," she says. "She was always on the verge of death. If I denied her and she died, then how would I feel?"

Dawn is now ashamed she was taken in for three years of fictitious suffering, as well as the tragic, unexpected "deaths" of Mandy's supposed real-life friends, Sophie and Pete, for whom she genuinely mourned. But Mandy put serious effort into her lies. When she posted as herself, she'd type as if she'd suffered a stroke, with lots of double letters and strings of words spelt backwards. Each of her personas had their own online identity and style. However much time Dawn devoted to comforting Mandy, Mandy must have spent more preparing for their online encounters. She emailed Dawn a photograph of herself apparently bald from chemotherapy, holding her baby son up to the camera. She even faked a seizure while they were having a webcam chat, convulsing with her eyes rolled back as Dawn looked on in horror.

The deceit only began to unravel because Mandy became greedy for Dawn's attention. "In the last six months it got a little silly," she says. "She was going from peritonitis to flesh-eating staph that was doing so much damage doctors were cutting flesh off her fingers and arms. There were too many dramas, too close together." Dawn wasn't prepared to share her growing doubts with the other forum members until she was completely sure. Nobody wants to falsely accuse a dying woman of lying. So she searched for Sophie and Pete's obituaries on Australian websites and found no record of their deaths. She looked closely at the photo of Mandy's bald head and noticed a five o'clock shadow. And when she went through Mandy's Facebook page, she discovered Mandy had been playing online games and commenting on friends' pictures at times when she was meant to be comatose or sedated.

Dawn confronted Mandy over Skype in March. At first Mandy played the role of wronged woman, but faced with Dawn's evidence, she begged for forgiveness. It fell to Dawn to tell the rest of the support group that the whole story had been a sham.

"She stole time from my kids and my husband, and I was able to be sucked in for so long because I couldn't fathom anyone making all this up for nothing. Why would someone want to hurt people like that? I'll never know."

MBI provides an explanation – albeit one that's been considered only by a handful of psychiatrists. They have plenty of cases to study. There's the anorexic mother and daughter who kept an eating disorders support group updated from the hospital where they were both in intensive care, until the forum members noticed they both consistently misspelled the same words, and revealed them to be the work of a lone faker. There's the 40-year-old single mother who posted for months on an ovarian cancer website until her 17-year-old daughter came online to announce news of her death, and her own tragic diagnosis with ovarian cancer. When her daughter's boyfriend informed the group that she, too, had died, someone smelled a rat and rang the local health department. There was no record of either death.

Then there's 18-year-old Limeybean, one of the internet's most notorious fakes, who said she was an immigrant living in London with a rare, untreatable form of tuberculosis. Limeybean blogged about her struggle on LiveJournal and garnered a wide following of concerned readers. When her death was announced on a friend's MySpace page, the LiveJournal community went into mourning. But some began to have doubts. One of her followers – a medical student – compared her account of the illness with what his textbooks said about tuberculosis. It didn't add up. When the community felt they had conclusive proof of her fakery, Limeybean returned briefly from the dead to resurrect her blog, only to delete it again and vanish.

"We can only guess why people do this, because rarely do the perpetrators come forward," says Dr Marc Feldman, clinical professor of psychiatry and author of Playing Sick. "Many of these people are simply after attention and sympathy that they feel unable to get in another way. These are people who often lack social skills, and they can't come up with more straightforward ways to ask that their needs be met."

Feldman is a specialist in factitious disorder – an umbrella term describing cases where people intentionally act ill, or claim their loved ones are ill, without obvious benefit to themselves. He coined the term MBI in 2000. "It's hard to say how common it is because the disorder is based in deceit. We're only detecting cases where the ruses have failed, which is probably a minority." Still, he hears of a new case every three weeks or so, and believes the phenomenon is becoming more widespread. "With the explosion of internet-based support groups and special interest groups, increasing numbers of people have realised these forums can be abused."

While it's impossible to develop a profile of the typical support-group scammer, there's little doubt that women are disproportionately involved, both as perpetrators and as victims. This is true of both MBI and factitious disorder in general. Online self-help forums tend to attract greater numbers of women than men, so women are more likely to be drawn into such a scenario in the first place, but Feldman offers another explanation. "Jails are full of men who get their needs met in direct, pathological ways, and women tend to resort to behaviour that elicits attention in an indirect way."

Jeanette Navarro is one of the few fakers who's prepared to talk about what she did. She's 24 and runs an online business from her home in the Philippines. She describes herself as an outcast, alienated from her family, with few friends. Jeanette does have a real medical condition – a rare autoimmune deficiency – but when she joined a worldwide online support group for fellow patients in 2008, she found herself exaggerating her symptoms and fabricating other personas to draw attention to herself.

"I was good at first. I didn't lie then," she says. "Everyone felt for me. Everyone was very sympathetic. It felt wonderful." Somewhere along the way, she says, she got "lost" amid the affection the group showed her. "I have never felt more loved and cared for in my entire life. I suddenly craved everyone's attention." After two months, she fell genuinely ill and took a break from the site for couple of weeks. "When I went back online, I found people were looking for me. That's when I posted as another person and told the group Jeanette was in a coma."

Her first lie was met with a deluge of compassion. Jeanette became intoxicated. "It made me feel so good, spending time with people who genuinely cared for me, even if they didn't know I was a fake." Once Jeanette started to lie, she found she couldn't stop. She'd spend 15-20 hours online a day, answering the 50 or so emails that arrived from concerned well-wishers, and ultimately invented five different characters to embellish and sustain the deception if attention moved away from her. Managing it all was demanding, she says. "My computer and desk were full of Post-it notes to remind me of what things I'd said to the group about a particular person."

Jeanette carried on for seven months, killing off two of her personas in dramatic fashion. But when she had time away from her computer she began to think about what she was doing. "It dawned on me I was playing with people's emotions. I started feeling guilty." Then she posted a long confession to the group, giving her real phone number, inviting people to vent their anger upon her.

The 200 people she'd been lying to were appalled, of course. "They said I was the lowest, most horrible, most despicable person that ever existed. They told me they wished I would just die," she says. Two years on, Jeanette often gets messages from group members telling her they're still devastated by what she did to them. She is now receiving psychiatric help, and takes medication for depression.

It can be almost impossible for online support groups to move on once they've found a faker in their midst, and some communities have been destroyed by the experience. They are supposed to be places of refuge for vulnerable people, and the trust on which they are based can be replaced by pervasive paranoia. If there's no outright confession, the group can be divided in two, with one side supporting the suspect and the other demanding their exclusion from the community.

With 1.2m visitors a month and 25,000 forum posts every day, Mumsnet has had its fair share of scammers abusing its message boards. One recent example involved a mother who'd been posting about her daughter's death in the bereavement forum, and was then found to have logged on using multiple identities, each with their own traumatic tale. After she was banned from the site, the women on the forum described feeling "violated", both by the suspected lies and by the witch-hunt that took place to try to expose them.

But Mumsnet's size makes it robust enough to bounce back from cases like this, and founder Justine Roberts takes a pragmatic approach to suspected scams. "It's part of our philosophy to take people at face value," she says. "We'd rather be taken in than deny support and advice to someone who's real."

Given that most fakers vanish at the first murmurs of doubt, there's little opportunity for their mindset to be studied by psychiatrists, or for them to get the treatment they may need.

MBI is a new concept, too new for the international psychiatric bodies who publish diagnostic criteria to have weighed in on whether it should be recognised as a distinct condition. But for those who specialise in factitious disorder, the idea seems very plausible. "It makes perfect sense that getting sympathy from potentially hundreds of people may be much more powerful than getting it just from one person in a white coat," says Dr Richard Kanaan, consultant psychiatrist at London's Maudsley hospital.

Whether feigning illness online or in the real world, fakers are often profoundly disappointed when they're told they may be ill after all. Many appear to prefer the stigma of being labelled cruel to that of being a psychiatric patient. According to Kanaan, this could be a false distinction. "There's confusion about where the line lies between being a bad person and being ill. Someone who's doing this, I'm afraid, could be both."

• Some names have been changed.