Attention seeking on the internet takes many forms, but the people who hoax online forums with tales of sick children are among the most painful, writes Jolyon Jenkins.

Little Charly Johns was a trouper.

She was only six years old and had cancer - but she fought it with determination. She was in and out of hospital as the disease advanced and retreated.

It was tough too for her mother Anna. She joined the Macmillan online cancer forum.

There she found support and help from people who knew exactly what she was going through.

For two years, Anna kept them updated on Charly's progress.

Find out more Jolyon Jenkins presents Desperately Seeking Sympathy on Radio 4 on Monday 4 June at 11:00 BST Listen via the Radio 4 website

"On the whole she is doing great," she wrote. "She is happy, lively, giggly and very easily excitable. She is always the first to laugh at anything and the last to stop. Nobody could look at Charly now and have any idea of the things she has endured these past 14 months."

But in November last year, Charly lost her fight for life. On the Macmillan forum there was an outpouring of grief. People wrote poems in Charly's memory. They painted their fingernails pink in accordance with her last wishes - even men.

But it was all a lie. Charly did not exist. Neither did Anna .

The whole thing was a hoax, discovered when the church in Paris where Charly's funeral was to be held turned out to have no record of her.

Faking it Munchausen syndrome sufferers pretend to be ill or induce symptoms of illness or injury to themselves

The term Munchausen syndrome by proxy (now known as fabricated and induced illness or FII) refers to people who invent or cause the symptoms of illness in someone else. The term is controversial, but the NHS maintains that a great deal of evidence suggests the condition exists

Term "Munchausen by internet" (MBI) coined by Dr Marc Feldman in 2000, after identifying a pattern in 1998, allied with rise of internet and chat forums

MBI sufferers construct false identities and feign illness or trauma in order to get sympathy or attention from online support groups

Sufferers often invent multiple online personalities to validate main character

The perpetrator, it transpired, was a teenage girl. The pictures of "Charly" were the girl herself when younger.

Many on the Macmillan forum refused to believe it. They had formed close online relationships with Anna. It seemed impossible that a teenager could have had such emotional maturity. Others left the forum in despair.

"These are some very desperate people," says Jackie Marshall, a member of the Macmillan forum. "People who may not have long to live, who are sharing burdens with complete strangers, because they are not comfortable sharing with families. The forum provides a lifeline."

It wasn't the first time Macmillan has been hoaxed, and Macmillan isn't the only forum to have been affected by impostors.

An American psychiatrist, Marc Feldman, has described it as "Munchausen by internet", similar to the well-known Munchausen syndrome, in which people fabricate illnesses to gain attention and sympathy.

It's no exaggeration to say there's an epidemic of MBI, and one which destroys the trust that underpins the forums.

Sometimes the hoaxes are astonishingly elaborate. "Cara", on the west coast of America, kept a blog in which she detailed her fight against cancer, as well as HIV, anorexia and heart problems.

We closely monitor the forums and make every effort to identify people who are falsifying a cancer experience Macmillan Cancer Support

She posted pictures of herself in a hospital bed wearing an oxygen mask and feeding tube, and a video in which she struggled to speak in the face of neurological difficulties.

She became close friends with two women, one of whom, Lauren, had Cara's name tattooed on her arm. All a lie. Cara was rumbled, and vanished.

Another of those taken in was Kaylin Andres, a fashion designer who kept a blog about her own cancer. They had Skyped, and Kaylin had sent Cara small gifts.

Then at the end of last year she logged on and read Lauren's account of her discovery of Cara's deception. "It was just such a bombshell. I remember reading it at work and I couldn't help but burst into tears," says Andres.

In recent weeks there are reports that Cara has re-emerged as "Mollie", a British teenager going through a problematic pregnancy. The hoaxers have a habit of popping up again in new guises.

The Allitt case Munchausen syndrome by proxy (now better known as fabricated and induced illness) came to public attention following the jailing in 1993 of nurse Beverley Allitt . Dubbed the "Angel of Death" by newspapers, Allitt murdered four children and injured nine others at Grantham and Kesteven Hospital. Allitt's actions were said to have been linked to the condition.

Pregnancy and childbirth provide a fertile territory for them. There are cases of women purloining each other's ultrasound pictures to post online, and of insinuating themselves into forums dealing with stillbirth in order to gather anecdotal details which they can then pass off as their own.

Strangest, perhaps, is the case of Rebeccah Beushausen in Chicago, who not only faked a pregnancy, but faked pictures of the newborn baby, using an ultra-realistic doll.

In that case, Rebeccah had ideological motives - as a Christian and anti-abortionist, she wanted to show that it was possible to carry a foetus to term even though it had a severe genetic disorder.

Feldman thinks that in general, MBI is essentially the same as "classic" Munchausen's but set on a new stage.

In some cases that's true. I spoke to a woman, "Amy", who hoaxed forums for years. She posted first as someone with cancer. Then later, even more bizarrely, she posed as a young girl who was being sexually abused. She was befriended by - and deceived - an older woman for six years.

But even before the internet existed, Amy had been faking medical problems. She dated it back to childhood.

Just as online fraudsters dream of easy money, these people crave easy attention

"When I was nine and a half, my little sister was born and she had cerebral palsy and I felt abandoned by my mother. So I faked an eye exam so I could get glasses like my sister had."

Amy is now seeing a therapist and thinks she has kicked the habit for good.

Another woman spoke of feeling unloved by her parents, of being a misfit, of finding that the forums were the only place where felt accepted.

But compared to classic Munchausen's, hoaxing online is easy. There's no need to fool medical professionals.

It gives the perpetrator a quick hit of attention, a feeling of being valued, but without really having done anything to deserve it.

Just as online fraudsters dream of easy money, these people crave easy attention. And it is, perhaps, just another form of fraud - emotional, rather than financial fraud. And emotional fraudsters are no easier to deal with.