The star-studded world AIDS conference will hear urgent appeals for funds at a crucial point in a war now in its fourth decade. The conference is opening in Washington.
Cash-strapped Western donors that have led a string of victories, in devising new drugs and getting them to poor people, are refusing to dig deeper into their pockets.
For some experts, this revives memories of the traumatic "money crunch" of a dozen years ago when millions of HIV-infected Africans were left to die.
On the other hand, domestic funds are now coming on stream in big developing countries -- as are smarter ways of using the money.
"After years of international investment, just when we seem to have the right technologies, drugs and approaches to keep the epidemic under control, success hangs in the balance," says the journal The Lancet Infectious Diseases.
In 2011, global spending on AIDS was $16.8 billion, an increase of 11 percent over 2010, according to figures published on Wednesday by UNAIDS.
The boost helped get another 1.4 million poor people on AIDS drugs, bringing the tally to a record of more than eight million, or 54 percent of those in need.
But there was a funding gap of $7 billion, half of it in sub-Saharan Africa, home to 69 percent of the 34 million people living with HIV.
For the first time, assistance from international donors -- the United States and other Western countries, together with philanthropists and agencies such as the Global Fund to Fight AIDS, TB and Malaria -- amounted to less than half of the worldwide contribution.
Their spending has remained almost unchanged since Western economies were whacked by the financial crisis in 2008.
Instead, emerging economies are shouldering far more of their own burden, which in turn is freeing Western funds for the poorest nations.
Brazil, Mexico and Russia now pay for almost all of their response to HIV; India has committed to paying for more than 90 percent; and China funds 80 percent of its domestic policies.
South Africa, which has the biggest number of HIV-infected people in the world, has increased fivefold its funding on AIDS over the last years. In 2011, it spent $2 billion, making it the top funder among low- and middle-income countries.
Yet even far poorer countries are raising domestic funds, often through innovative ways.
They include a tax on mobile phone calls in Rwanda and Uganda, and plans in Kenya and Zambia to establish an "AIDS trust fund" that would pool a variety of revenue streams, including a ring-fenced contribution from the government.
Hand in hand with the search for money is an effort to squeeze more out of every dollar.
All-in-one treatments, use of health workers to substitute doctors and nurses to distribute pills and prevention of infection, especially through male circumcision programmes in Africa, are getting a special push.
"There is a sense of more for less, of greater bang for the buck," said Awo Ablo, director of external relations at the International HIV/AIDS Alliance, a network of grassroots groups.
"There is a great emphasis on efficiencies, in terms of programming, and also applying and learning from operational research."
Yet even with this, the money problem will linger, she said.
The more lives that are saved the higher the immediate bill, for anti-retroviral drugs have to be taken daily for the rest of one's life to keep HIV at bay.
To provide drug access for all 15 million badly-infected people by 2015 under a 2011 pledge, the world will need $24 billion that year, according to UN estimates.
By 2020, the annual bill should decline to $21.5 billion, because antiretrovirals not only treat infection but also cut the risk of handing on HIV to others. The number of new infections last year was 21 percent below the pandemic's peak, in 1997.