The biggest global epidemic in recent years? It would have to be swine flu, or H1N1, which has killed 18,239 people already, right? Wrong. That is just the one you have heard most about in recent years because it's new, and that makes it newsworthy.

The world's biggest infectious killer is HIV-AIDS, says the World Health Organisation. It's killed 27 million people so far. But isn't it history? Isn't AIDS a 1980s-'90s thing? The disease was first diagnosed in 1982 and a breakthrough in anti-retroviral drug treatment came in 1996. Hasn't that crisis passed?

It's true that the official rate of new infections worldwide is falling. It peaked with 3.5 million new cases a year in 1996. It then trended down gradually to 2.7 million in 2008. And we know that the spread of the disease is entirely manageable. Australia is a good example. With the right public information, the right strategy and enough funding, Australia has controlled its spread. Only 0.1 per cent of Australians carry the virus. It's the same story with all the world's rich countries, with the notable exception of the US. With six times Australia's rate, it has the same prevalence of HIV-AIDS as Somalia and Costa Rica.

But the real HIV-AIDS crisis may still be ahead of us. And one of the biggest reasons is the complacency that has settled on world public opinion, the idea that it's all under control. There are five chief reasons that the threat is looming as large as ever.

While the downward trend in global averages for new infections is apparently reassuring, that average conceals the fact the epidemic is growing explosively in some countries.

While it's raging in sub-Saharan Africa and eastern Europe, the number of new infections is also increasing in places closer to home. It's rising in large swaths of Asia: in parts of China, in Vietnam, Cambodia and Thailand, and also in the South Pacific, notably in Papua New Guinea.

The epidemic is evolving, too. In China, for example, it was primarily a phenomenon among people injecting drugs, spread by dirty needles. Now, it's increasingly spread by heterosexual couples having sex, as the United Nations' dedicated body, UNAIDS, has reported.

In some countries, there is a total lack of even the most basic understanding. Consider the amazing findings of a serious academic study in Pakistan in 2007, for instance.

Incredibly, 28 per cent of prostitutes had never heard of HIV or AIDS. And 60 per cent didn't know that a simple condom could stop the spread of the disease. This state of affairs in many countries practically guarantees that there will be flaring epidemics in the years to come.

The second reason is that when HIV-AIDS spreads, it spreads more than HIV-AIDS. Much as the initial damage from a nuclear blast is soon followed by a shower of radioactive fallout, HIV-AIDS trails behind it another wave of death and suffering in the form of tuberculosis.

Once AIDS sufferers' immune systems are broken, TB is the disease most likely to follow. And this creates an ever-expanding pool of TB sufferers. Remember that TB is far more infectious than HIV-AIDS and can be transmitted with a simple cough. It's also developing into drug-resistant strains for which there is no known cure. To manage the spread of TB, the world needs to control HIV-AIDS.

Third is the fact that, thanks to the anti-retroviral drugs that keep the infected alive, there is an ever-expanding population of people living with HIV-AIDS. Some 33.4 million people now live with the virus. As this population continues to swell, the cost of supplying anti-retroviral treatment will mount year by year, putting an increasing demand on government and NGO budgets alike.

Fourth is the crisis in funding for HIV-AIDS. There are two big sources of money to fight the disease. One is the Global Fund, financed by the world's rich countries and some big private philanthropists, notably the Bill & Melinda Gates Foundation.

This fund has had a cumulative total of $US19 billion poured into it since it was founded in 2002. It estimates that it needs a minimum of $US13 billion more to continue its work over the next three years. A big donor conference is due in October, and the outlook isn't good.

"The global financial crisis rocked all the donor countries, but what's really rocked them is the second wave that's now going through Europe," where governments find themselves in fiscal crises, says the executive director of the Pacific Friends of the Global Fund, Bill Bowtell.

"The European governments have been fantastic donors in the past, but it seems very unlikely that they can repeat the performance this year, so the outlook is not good," says Bowtell, who was the policy adviser who developed Australia's successful HIV-AIDS policy for the Hawke government.

The other big source of funds is from the US government, the President's Emergency Program for AIDS Relief, which George W. Bush founded with an initial allocation of $US50 billion. It faces a similar problem. "That fund will need a new allocation in the next four or five years," says Bowtell, but with the budget crisis now gripping the US "it's hard to see the US Congress will authorise the money".

Bowtell wants Australia to help fill the global funding gap, and he hopes China will, too. "Until just a few years ago China was a recipient of international money for HIV-AIDS programs - today it could easily make up the shortfall from Europe at the blink of an eye."

The final source of a re-energised HIV-AIDS pandemic is government wrong-headedness. "Wherever sex or injecting drug-users are criminalised or stigmatised, the AIDS epidemic is fuelled," says Bowtell. Because the people who most need screening and treatment will be reluctant to step forward if they risk arrest or social stigma. So the disease spreads. In China, for instance, where drug-users are punished severely, it's estimated that only one-third of carriers have been identified. And 85 countries still have laws against homosexuality, seven of them with the death penalty.

That is why a big HIV-AIDS conference in Austria later this month will issue the Vienna Declaration: "The criminalisation of illicit-drug-users is fuelling the HIV epidemic and has resulted in overwhelmingly negative health and social consequences. A full policy reorientation is needed." That is why, far from being a relic, HIV-AIDS could yet be a 2010-'20s thing, and beyond. In the meantime, it's killing another 2 million people every year.

Peter Hartcher is the Herald's international editor.