In the 20 years since the Aids pandemic began, it has claimed the lives of 20 million people.
Sex and religion: Vatican policy is based on fidelity
Many agencies working in the developing world believe that increasing the use of condoms globally would dramatically cut the death toll.
Yet the Catholic Church, a powerful presence in many of the worst-affected countries, continues to oppose such campaigns.
The Vatican says that encouraging people to use condoms is immoral, and far from checking the disease, will help it spread.
In Kenya, the archbishop of Nairobi argues that condoms are "a licence for sexuality" and make young people feel they are safer than they actually are.
The implacable stance of the Pope has dismayed many doctors and scientists.
However, the Catholic Church is heavily involved in caring for those affected by HIV and Aids in many parts of the world, and some within the Church now question the doctrine.
The Vatican's point of view is rooted in arguments aired in 1960s.
Following the introduction of the contraceptive pill, Pope Paul VI issued his encyclical Humanae Vitae.
At the time, many Catholics hoped it would sanction birth control. But the Pope ruled that nothing should prevent the possibility of a child being created from the act of sexual union, which was to take place only within marriage.
Today the message remains the same. Under the papacy of John Paul II, the Vatican has upheld a view of sexual morality based on fidelity and family values, and abstinence outside marriage.
But the spread of HIV/Aids since the 1980s has made the use of condoms a major public health issue in many Catholic countries.
In Brazil, where half a million people are HIV positive, the issue has brought the Church into conflict with a government trying to promote safer sex through the use of condoms.
It has also created tensions within the Church itself, as senior clerics have tried to enforce the line from Rome, while individual priests sometimes take a more pragmatic view.
'Fight for life'
Father Valeriano Paitoni, an Italian priest who works with Aids patients in Sao Paulo, is one of the Vatican's fiercest critics.
"Aids is a world epidemic, a public health problem that must be confronted with scientific advances and methods that have proven effective," he says.
"Rejecting condom use is to oppose the fight for life."
While some Brazilian bishops are thought to be sympathetic, Paitoni's words brought a swift rebuke from the city's archbishop, Cardinal Claudio Hummes, a man tipped by some to become the next pope.
And there is no doubt that the policy comes from the top. In October, a senior Vatican official told the BBC's Panorama programme that the latex rubber used to make condoms contained "pores" that would not stop the transmission of the HIV virus.
The World Health Organisation said the claim was not only untrue, but was also dangerous. It stresses that virus particles do not pass through latex.
The Vatican sees no reason to retract the statement. "You cannot talk about safe sex," said Cardinal Alfonso Lopez Trujillo, who heads the Pontifical Council for the Family.
"One should speak of the human value, about the family, and about fidelity."
Safe or safer?
The World Health Organisation does say, however, that sex with a condom is not 100% safe because of the risk that the condom will not be used properly or will break.
This is a concern which many abstinence advocates emphasize.
Cardinal Hummes: Enforcing the Vatican line on condoms
Presbyterian Fernando Chomali, the adviser on ethical issues at the Chilean Episcopal Conference, says:
"When [international organisations] say that the way of stopping Aids is to use a condom, they do not say that it is 100% safe. It's so important that they have changed the language they are using. They started talking about safe sex, now they are talking about safer sex."
The argument that the disease can only be defeated by persuading people to change their sexual behaviour has found an echo in the United States.
Washington has promised to spend $15 billion over the next five years on tackling HIV/Aids around the world. One third of the money is being allocated to projects promoting sexual abstinence.
However, some members of the Catholic clergy are adding their voices to those questioning Vatican policy. Kevin Dowling, the Bishop of Rustenburg in South Africa, says the Church needs to recognise the reality of Aids.
"I believe it demands a re-think in terms of our traditional approach to moral theology, ethics and to the preservation and protection of life," he says.
"I believe we must give people correct information on condoms all scientific evidence shows that the proper, sustained use of condoms can significantly reduce the infection rate."
Catholics who support the use of condoms to save lives argue that it is the lesser of two evils.
But for the Vatican, the acceptance of condoms would be a denial of the long-held belief that sex is all about procreation within marriage.
It is a doctrine that is now being challenged by the global tragedy of Aids.