Luc Montagnier, who led the team at the Pasteur Institute in Paris that discovered the virus, said a reappraisal of HIV can eventually help scientists find a cure. He said: 'It is possible. I'm optimistic because of what I know from my own research into how the virus can affect immune suppression. There are grounds for new hopes of making a vaccine.'
Developing drugs that can cure Aids patients and protect infected people against disease may even be feasible 'if we have the means of eradicating infected cells', he said.
Professor Montagnier's optimism is not, however, universally shared. One of the most difficult unresolved issues is understanding the precise way that HIV causes the breakdown of the immune defences, leaving the body open to invasion by potentially lethal infections that are normally fought off with ease.
Jim Neil, a virologist at Glasgow University, said: 'The immune system is complex and therefore how HIV leads to the decline and fall of the immune system is going to be complex.'
Also, scientists are studying a disease that does not exist in laboratory animals. The only parallel is a related virus - SIV - that infects certain species of monkey.
Researchers trying to develop an HIV vaccine have first to try out their ideas using SIV. Some scientists believe the parallels, however, are too distant for developing a reliable human vaccine.
Apparent successes in developing prototype vaccines that seem to work with SIV in monkeys have not been matched by equal success with HIV. Martin Cranage, who is trying to develop an Aids vaccine at the Government's Centre for Applied Medical Research at Porton Down, said past work on vaccine development had been 'a roller coaster' with lots of ups and downs.
Attempts to develop drugs that combat HIV have met with similar problems when tested on human volunteers. The failure of the anti- Aids drug AZT to live up to its initial promise has forced scientists to rethink their strategy for designing effective treatments.
Virologists have shown that the great ability of HIV to mutate inside the body enables it to develop forms that are resistant to AZT. These strains eventually overwhelm the drug and cause Aids. One alternative now under investigation is to combine a number of anti-HIV drugs to make it more difficult for the virus to develop drug-resistance.
This strategy, however, is prone to side-effects. Drugs such as AZT that are effective against HIV for a while are often toxic when taken over long periods.
Anthony Fauci, a leading American Aids researcher and director of the US National Institute of Allergy and Infectious Diseases near Washington, said the combination of a virus that can hide in human chromosomes, that attacks the immune system and that mutates rapidly has created 'an unbelievably unique situation'.
No other virus attacks the immune defences, 'the very system that under any other circumstances would eliminate the virus from the body and protect the individual from other exposures to it', Dr Fauci said. He does not talk in terms of a cure, but of the possibility of new treatments that will progressively prolong and improve the life of patients.
Abraham Karpas, a virologist at Cambridge University who was the first to isolate HIV in Britain, is also deeply pessimistic about the prospects of a cure or a vaccine. 'We know more about this virus than probably any other. We know practically everything but we don't know how to stop the virus from doing progressive damage to the immune system which inevitable ends up with Aids in the majority of infected individuals.'
Vaccines, he said, will not work primarily because of the way HIV gains entry into the body by infecting cells of the immune system which can slip past the defences set in place by innoculation.
'The future is in education and prevention,' Dr Karpas said. 'On our present knowledge, I cannot see a cure or vaccine in sight.'