Thirty years ago, in New York and San Francisco, a small number of young men became inexplicably and very seriously ill. Some had a particular cancer while others had a form of pneumonia that had never before troubled that age group. Their immune systems were shot, their bodies unable to fight back, and they died. They were the first documented cases of Aids, a new disease that would terrify entire populations as it scythed down rich and poor, celebrity and nonentity. Rock Hudson. Freddie Mercury. Arthur Ashe. And thousands whose names were known only to those who loved them.

Today, on World Aids Day, the disease is still incurable – but not untreatable. After years of intense scientific effort, huge sums of money, and some of the most effective health campaigning ever seen, people who become infected with the HIV virus can live normal lives, albeit on daily medication. In the more comfortable regions of the UK, the US and Europe you hardly hear about it. Yet among the deprived and the marginalised, the numbers are still growing.

Sub-Saharan Africa has been the campaigners' focus for more than 10 years. To everyone's enormous credit, the rate of infection and deaths has slowed as drugs that used to be the sole property of rich countries have been rolled out in cheaply manufactured versions to the developing world.

The great news this year is that scientists have now tentatively offered us a way to end Aids. Studies in recent months have shown that the drugs that keep people alive also stop them infecting others. A man who is on a standard combination of three antiretroviral drugs is 96% less likely to transmit HIV to his partner. That news has fired up all those working against Aids.

Three weeks ago Hillary Clinton, the US secretary of state, stoked the excitement, offering to lead the world towards the goal of "an Aids-free generation". Politicians pick their fights. Science, she said, had shown this one is winnable, with drugs to prevent people infecting their partners, the same drugs to prevent mothers passing the virus to their babies in childbirth, and male circumcision, which also reduces the risk for men.

But in the last week the champagne bubbles have gone flat. The Global Fund to Fight Aids, Tuberculosis and Malaria, launched by the UN secretary general Kofi Annan a decade ago, has cancelled its next funding round. Developing countries need not apply – there is no cash. A year ago, when the fund asked for $20bn, donors coughed up just $11.7bn and the money has not been much boosted since. If this were not so deadly serious it would be absurd. As Clinton declares the end of Aids is nigh with one massive last push, the donor governments, mostly in Europe, sit on their wallets. HIV/Aids has gone out of favour. It is said to have had too large a share of the cake in years gone by – although perhaps the overall cake for developing world health would have been smaller without it.

Europe's economic crisis is also to blame, of course, and so is bad publicity over some grants that fell into corrupt hands in Africa – although reforms are under way and the UK still gives the fund an A1 value-for-money rating.

But what is being overlooked as donors quit, claiming they have already done enough, is not only the opportunity of an Aids-free generation, but also our moral responsibility to those now on treatment and those who need it.

Take Malawi, a country that has performed wonders against HIV/Aids. More than 90% of its funding comes from the Global Fund. It has managed to put 76% of all those who need it on treatment, decentralising care so that nurses instead of doctors can start administering the drugs. It wants to start all pregnant women on drugs for life, instead of offering them a short course around childbirth.

Malawi had its grant application turned down last year. Cash-strapped even then, the fund said its plan was too ambitious. And there will be no grant approval next year, either. Suddenly there are real fears. Those people already on drugs must stay on them – or their virus will become resistant and they will need new, more expensive drugs to stay alive. The fund is talking about provision for "essential needs", but nobody knows what that means.

Zimbabwe, Kenya and Congo are among the others whose plans to put more people on drugs will fail without money. How can this be the right time to let the Global Fund founder? There are more than 6 million people in poor countries on the drugs now – but just as many are still in need, and many more will soon join the waiting lists. Without treatment and continuing effort, three decades of progress could be reversed.