'It is hard for me to get my head round. Why should they listen to me?'

This is a story about fame. Last year, three men fronted a video and poster campaign to try to reduce the spread of HIV in Sierra Leone. They told young men to wear a condom, to stop blaming others for not getting themselves tested, and to fight the stigma of the virus. The campaign achieved dramatic increases in the number of young men who knew how to practise safe sex and who were prepared to be tested. However, the people giving the advice were not doctors, politicians or even film stars, but footballers: Ryan Giggs, Rio Ferdinand and Patrice Evra.

"It is weird," said Michael Carrick, who is talking to teenagers in Hangzhou about the stigma of Aids as part of a Unicef project. "You see kids who maybe haven't got long to live, they meet you and they are happy for a while. It is hard for me to get my head round that. Why should they listen to me? I find that so strange."

It is a good question. Hangzhou is a modern, prosperous city to the west of Shanghai, and Manchester United are big news. Their hotel, on the shores of a mountain-ringed lake that gives it the feel of an oriental Switzerland, was constantly under siege; the moment someone in club uniform appeared on the stairs, hundreds of voices in the lobby exploded. Some fans have slept on the pavement outside.

However, Aids in China is largely a rural phenomenon, with the virus most rampant in Yunnan province, near the border with Burma, and it is spread by the drugs of the Golden Triangle not sex. And surely nobody in Yunnan knows who Michael Carrick or Dimitar Berbatov are?

Yet the Unicef officials in the Hyatt Regency tell a story of one of their colleagues who travelled to a desperately remote village in Trinidad that she thought had been abandoned. Eventually, she found everyone in a single house, crammed around a television watching the European Cup final. Manchester United have never played in Sierra Leone and, given the poverty of the country, are unlikely ever to do so. Most people in Yunnan have television, and the name Manchester United will mean something to someone.

"Listen," said Berbatov, who, like Giggs, is a Unicef ambassador and supports five care homes in his native Bulgaria. "If David Beckham says something about HIV, then people will remember it. People do listen to the famous and maybe that is a little bit strange. When I was a boy I listened to my parents because what better example can you get than your parents? But many of these kids don't have parents; they need someone else.

"But really, I don't think you should need to talk to a famous person before you can address the problems you face. You need to understand the problems before they hit you. We come here for a reason, that is to train and play football, but there is also this, and I would say it is equally important and when I look back on my life it might be more important."

Nearly six out of every 10 Chinese teenagers have no idea how to protect themselves from Aids. Sixty-nine per cent believe it can be passed on through chopsticks. Unlike in South Africa, where Manchester United toured last summer, the Chinese government has been quick to recognise the spread of the virus. It is illegal to discriminate against HIV carriers, but to emphasise the point Berbatov and Carrick take part in a game where each teenager has a sticker placed on their forehead. They don't know what colour it is. If it's blue, everyone can shake hands with them. If it is yellow, you can go up to them but you can't shake their hands, although they can touch yours. If you have a red sticker, nobody goes near you. These 12- to 15-year-olds with red stickers quickly discover what it is like to be shunned.

Games like these, along with books, educational materials and sports equipment, are packed in boxes and sent to 295 schools in the worst-affected areas. It is a two-year project and the £230,000 cost is borne by Manchester United. The box itself opens up into a sort of blackboard. It trades under the name of "Skills for Life in a Box".

It is easy to be cynical about footballers and these kinds of events. You think of Cristiano Ronaldo constantly looking at his watch during the club's media day to mark the 50th anniversary of the Munich disaster; of Liverpool's reserve keeper Charles Itandje giggling through the service to mark the 20th anniversary of Hillsborough. It is a box to be ticked, like listening to Harry Enfield's creations, Smashie and Nicey, talking about "charideee".

But it matters to others. CLR James, the great writer on the Caribbean, asked: "What do they know of cricket who only cricket know?" It goes for football, too. "But if it wasn't for football, I wouldn't have seen the things I have," Carrick reflected. "I am being educated. I have been to hospices and orphanages and if not for football, then I would never have done.

"It makes a massive difference now that I have a child of my own. I went to South Africa last year. It was my first tour and when I got home I thought that having a child changes a lot. I come here, I see these children and what they are going through and look at it in a completely different light.

"It puts into perspective how lucky I am, how lucky my little girl is. She has things, she has her health. But it feels surreal being here, trying to help but you just keep thinking ..." And he can't put it into words.