Pamela Anderson and Pope John Paul II may seem unlikely bedfellows, but they have at least two things in common: both are world famous, and both have personal experience of potentially fatal diseases.

Pammy's recent claim that she contracted hepatitis C by sharing a tattoo needle comes 10 years after the Pope had a bowel tumour removed. But while the Pope has blessed, and aligned himself with, the International Digestive Cancer Alliance, the former Baywatch beauty appears still to be up for grabs for organisations raising awareness of hepatitis.

Any such organisation wishing to avail itself of Anderson's services had better hurry, though - celebrities can have a dramatic impact on awareness of health issues and fundraising for medical research, and competition for their support is fierce.

A celebrity's relevance to the cause is key: hence Victoria Beckham's support of the Meningitis Research Foundation - she is the mother of a young child, and has experienced viral meningitis herself.

Those recruiting celebrities also need to be well informed. Janice Mur of JMP, a PR agency specialising in health-awareness campaigns for charities and government, explains: "It's essential to strike while the iron's hot. When a new face comes on the scene - or an old celebrity makes a new health-related revelation - we're on the case immediately, though obviously we take care to raise matters delicately."

But even though a hasty approach could cause offence, being too sympathetic can leave you high and dry, as the British Heart Foundation (BHF) discovered when Ulrika Jonsson's newborn baby was diagnosed with a serious heart condition.

"Our medical director advised us that Ulrika could be highly traumatised, and forbade us from seeking her support for at least six months," says Jo Hudson, a press officer at BHF. "But within a couple of months, Ulrika appeared in the press expressing her concerns about her child's future and pledging support to Sir Magdi Yacoub's 'Chain of Hope' charity."

BHF stand by their decision, but admit that Jonsson would have been highly appropriate, particularly since, like most health charities, they are picky about the celebrities they use. TV presenter Gabby Logan, whose younger brother died from a genetic heart disorder, works well for them, but smokers - and anyone else whose lifestyle contradicts BHF's key healthy heart messages - are strictly off-limits, regardless of their profile.

The Institute of Cancer Research's Everyman charity had no such qualms when it accepted Robbie Williams's offer to parade around Malibu wearing false breasts in its memorable 1999 TV campaign. Despite his smoking habit - the cause of 95% of lung cancers - Everyman felt his profile was too high to resist in their quest to raise awareness of the 96% cure rate for testicular cancer caught at an early stage. Sure enough, calls to Everyman's testicular cancer helpline rocketed after the ads aired, and the charity knows of at least one man whose life was saved.

The institute is also home to the UK's first dedicated breast cancer centre, funded to the tune of £15m by the charity Breakthrough. According to Breakthrough's celebrity co-ordinator Stuart Barber, around a fifth of the "bricks and mortar" funding for the research centre comes from celebrity involvement with the Fashion Targets Breast Cancer campaign, launched this week for the fourth time since 1996.

"Fashion Targets is our largest and most visible campaign and relies entirely on celebrity support," Barber explains. "We publicise pictures of well-known models wearing campaign T-shirts. Women see them, buy the T-shirts, and all proceeds go into breast cancer research. To date we've sold 200,000 T-shirts, raising £3m. This year we're aiming to raise 20% of our target income from Fashion Targets alone."

But the campaign has also attracted criticism. In 1998, the Cancer Research Campaign suggested that using young, glamorous celebrities to raise awareness of a disease that predominantly affects women who are more than 50 could create a generation of neurotic teenagers. Since then, Breakthrough have taken care to mix older people such as Linda Gray and Jane Asher with the likes of Helena Christensen and Samantha Mumba.

Many health charities would kill for such support, and Barber is at a loss to explain why celebrities flock to certain causes, such as breast cancer, while less "sexy" health issues such as heart disease or mental health tend to receive less support.

Sue Baker, head of media relations for the mental health charity Mind, admits frustration at the dearth of celebrities willing to campaign on behalf of the issue. "Celebrities can definitely help to reduce the stigma attached to mental health problems. Even though we keep abreast of anyone reportedly experiencing mental health problems, we'd never 'out' anyone or publicise their experience without permission. The problem is that their permission is hard to come by - we can count our active celebrity supporters on one hand."

Perhaps she could learn from Puja Morjaria of the Forster Company, who persuaded John Hannah and Patsy Palmer to participate in the government's Mind Out for Mental Health "1 in 4" photography exhibition. "We asked celebrities if they'd be photographed not because of their celebrity status, but because they had experienced mental health problems. The fact that their pictures were exhibited alongside images of ordinary people was highly appealing to them."

Better still, to really ram health messages home, awareness-raising organisations could do worse than follow a recent trend used in some South American countries to publicise the unfortunate side-effects of smoking. They used dead celebs.