Everybody loves Yvette. The 12-year-old from Bristol has an irrepressible sense of mischief that affects eveyone who meets her, but her friends, and her family, cannot save her life: for that, she needs a stranger.

Without a bone marrow transfusion, Yvette Gate will die. Her body needs constant medical support to fight the slightest infection. Since her disease, a rare condition called aplastic anaemia, set in more than a year ago, she has been unable to go to school or mix with other young people. This week she lay in an isolated bed at Bristol Children's Hospital after doctors were forced to remove a catheter tube from her chest, which had become infected.

But despite the pain and the loneliness, she smiles for everyone. Her mother says: 'That child is always positive. When we look in her eyes, it gives us confidence. So I stay positive - I'll never give up.'

When a white British child needs fresh bone marrow, the likelihood of finding a donor with a tissue type that matches exactly is high. The Anthony Nolan Trust, an independent charity that maintains the UK's largest bone marrow donor register, estimates that matches can be located in 70 per cent of cases.

But Yvette is black and her tissue type is uncommon. Her parents, David and Mary, have been told this means that the chances of a match are as low as one in 100,000. She needs a miracle.

Bone marrow generates blood, including the white cells that fight infection. Its type is determined by six genes, which act like a six-digit combination lock. If a transfusion is given and just one of those digits is out by a fraction, the new tissue will react as if the patient's whole body is one colossal infection.

Even Yvette's older brother, Solomon, does not have the right genes to unlock that combination. David and Mary are searching for a match the only way they can, by approaching strangers, especially from Bristol's black community, and pleading with them to register as potential donors. By setting up their stall at street festivals such as the St Paul's carnival, and issuing appeals through local press and TV, they have signed up 100 people in three months. It's a faint hope, but it is hope.

'When I first started talking to the doctors I would be shaking and crying,' Mary says, 'because it was all so negative. I used to be really quiet, but now I'm tough.'

Yvette's chances are hampered by the lack of registered donors from ethnic backgrounds, the genetic diversity of black Britons - in part, a legacy of slavery - and the reluctance of people to volunteer for a procedure perceived as painful, even dangerous.

'The fear factor is quite shocking,' says Orin Lewis, co-founder of the African Caribbean Leukaemia Trust.

Lewis, whose stepson Daniel De-Gale's life was saved by bone marrow from a Detroit woman, is fighting to change attitudes 'by any means necessary. Our young people have their ears pierced, their noses and tongues and belly buttons pierced, they'll have tattoos. But ask them for a blood sample and they say, "No, I don't like needles".

'The first step is just a jab, to take maybe a teaspoon of blood. From that, doctors can tell your tissue type. It might be years before you're asked to make a donation, if it ever happens. Daniel's cousin, Kamisha Guthrie, signed up in 1997: this year, she saved a life.'

Last week Lewis delivered a lecture at Belmarsh prison, telling his audience: 'When you get out of here, the first thing you can do is help give life to a stranger. And if you aren't going to get out, tell your girlfriends when they visit, tell your brothers and sisters. The black community has a tradition of helping each other, with housing, education, everything. Don't let that spirit die.'

Daniel puts it succinctly: 'You say "bone marrow transplant" and the first thing people hear is "bone". That's it - they don't want to know. We need to make people understand that, for the donor, it's just a half-hour operation. When they wake up, they feel some tightness in the back, but it isn't painful. Most people imagine it's agony, but that's a myth.'

Daniel's donor said she felt as if someone was pinching the muscles in the small of her back; Kamisha said she'd had worse pains with her periods, and others compared it to the muscular ache after a morning's gardening.

Reluctance to volunteer as a donor is far from being confined either to ethnic communities or to the hunt for bone marrow matches: it afflicts every area of society and of healthcare in Britain. About a quarter of the population is on the NHS organ donor register, yet last year more than 400 people died while waiting for a transplant and 6,000 people are on the waiting list for a replacement heart, liver, lungs or kidneys.

The bone marrow tissue of black Britons is exceptionally diverse: for many people of African descent, their ancestors were forced from their villages and thrown together on the other side of the world. Genetically, it was as if hundreds of narrow streams converged in a reservoir.

'Doctors sometimes tell us a new black donor has some tissue type they've never seen before,' Lewis says. 'That's great for research, not so great when you realise how diverse we are.'

Dr Paul Travers, deputy research director for the Anthony Nolan Research Institute, based at Royal Free Hospital, echoes the metaphor: 'Finding a matching donor is essentially a lottery and each donor in the register is a potential winning ticket ... the more tickets you have, the greater your chance of winning. Three per cent of Anthony Nolan's register of potential donors are from the black community. The problem is those numbers are, in real terms, very small.'

For David and Mary, this slew of statistics are just part of the nightmare. 'Yvette was 10 when she started getting nosebleeds and bleeding from her gums,' says David, 41, a-services manager at the University of the West of England. 'Doctors warned us after the first few tests that she might have leukaemia. When they said it was aplastic anaemia, we thought, "That doesn't sound too bad". But it's worse because, compared to cancer, this is a mystery illness.'

David taught himself web design to put Yvette's campaign on to the internet and encourage children to send her emails. 'She's got pen pals from all over,' he says. 'There's one in Nashville, Tennessee. That's fun for her, but what she likes best, when she's well enough, is to put make-up on people. She's got a real gift and wants to be a make-up artist when she grows up.'

He straightens the teddy, dressed in a nurse's uniform, on his child's bedspread, and touches her head. For a moment, he and Mary are struck silent by the implications of what he has just said. Then Yvette starts smiling.

What you can do

Contact the Anthony Nolan Trust on 020 7284 1234 or 090 1882 2234

Register as a bone marrow donor by going to King's College Hospital in Camberwell, south London, between 10am and 4.30pm, and signing up at the boardroom in the Hambleden Wing

Get an organ donor card at Uktransplant.org.uk

Send a message of support to Yvette at admin@yvettegate.co.uk