In the early 1980s scientists isolated HIV and established it as the cause of Aids. The discovery paved the way for an international effort to defeat the disease.
It was said that an effective vaccine would be developed in five years. Yet 20 years on the results of the first large-scale trial of a candidate vaccine were disappointing.
Despite two decades' work on HIV, a vaccine is still years away
Earlier this year the US company VaxGen announced that their vaccine reduced infection by less than 4% - which was considered statistically insignificant.
It did, however, seem to be more effective among some ethnic groups, reducing infection rates by 67% among Asians tested, and 78% among black volunteers.
These results give a glimmer of hope, but they should be treated with caution as the numbers involved were so small.
Attention has since shifted to the Oxford-Nairobi vaccine currently being developed by a group led by Professor Andrew McMichael of Oxford University.
His team are fine-tuning the dose and methods of administering the vaccine before tests on a few hundred volunteers in Uganda and the UK, which they hope to begin in 2005.
If these go well, larger scale trials will follow, meaning any effective vaccine is still considered to be at least five years away.
To most people vaccines are injections that prevent infection. This vaccine will certainly not achieve that.
Vaccines can, however, help boost the immune system of those already infected, helping them fight the virus and extending their lifetime. It is this, at best, that the vaccine will achieve.
"This vaccine isn't going to be perfect - though it'll mean we'll have made progress and after that we can push a bit harder," says Professor McMichael.
Most of the vaccines that are currently under development are most likely to help only a proportion of those inoculated.
Professor McMichael hopes that his team's vaccine will help up to two-thirds of those vaccinated.
But there are likely to be ethical problems if vaccines help only, say, one in three people.
Some believe that it would be difficult not to offer a medicine that could do some people some good, yet in the long term it may give false hope and lead to complacency.
One of the reasons it is so difficult to develop a vaccine is that the virus itself keeps changing.
According to Professor Neil Fergusson of Imperial College in London: "The problems are immense when HIV mutates. We can't keep changing the vaccine every year.
Researchers are trying to develop better drugs that slow down the virus
"We still have an incomplete understanding of how HIV causes Aids - until we know that we won't be able to eliminate it. All we can do is slow it down."
The best hope is to have a vaccine which raises the body's immune response to several parts of the Aids virus - preferably parts that do not change too quickly.
The Gates Foundation has set up a special fund to boost basic research into global health problems such as HIV.
The foundation has invited bids of up to $20 million for research proposals which investigate ways round some of the scientific blocks to progress.
One of the key obstacles to the development of new therapies is the pressure to reduce the cost of drugs to poorer countries.
Economically this reduces potential revenues for drug companies - which reduces the amount available to invest in developing new and better treatments.
"It has become apparent at recent Aids conferences that there are far fewer new drugs beginning clinical trials than there were five years ago," says Janet Darbyshire, the Medical Research Council's director of clinical trials.
As well as vaccines, researchers are trying to develop better drugs that slow down the virus in those who are infected.
Drugs developed in the 1980s and 90s have been tremendously successful in enabling those infected to live healthier lives for longer.
But new classes of drugs are needed which are less toxic and able to tackle new strains of the virus which have become resistant to existing treatments.
Since the mid-nineties only one new class of drugs has come through clinical trials.
Professor Anderson of Imperial College, an adviser to the board of the Gates Foundation, says more money is needed:
"It seems a politically incorrect thing to say but the falling price of drugs is beginning to result in major pharmaceutical companies investing significantly less in research to develop the next generation of HIV drugs."
"There must be a sensible pricing policy to ensure that cheap drugs in Africa are offset by higher prices for the richer nations. We urgently need new drugs to prevent the inevitable evolution of drug resistance negating the great value of therapy."