Vaccine is now being developed to help cells hit back

It is hoped the discovery will be the key to developing more effective drugs and vaccines to treat and prevent Aids.

The research, reported in the journal, Science, is in its early stages but scientists at the Institute of Molecular Medicine in Oxford believe they have made a real breakthrough.

BBC Science's John Newell reports (0'41')

HIV infects white blood cells, which normally protect people against disease. It weakens the immune system to such an extent that patients often die of infections they would normally be able to fight off.

But scientists have discovered that some uninfected white blood cells do try to strike back against the disease.

The cells identify and destroy infected cells, killing the virus at the same time.

Researchers have shown that although the virus usually wins in the end, the more fighter cells there are, the more effectively it is destroyed.

Human trials are unlikely to be possible for several years

They are now working on new types of treatment designed to stimulate this type of white blood cell against HIV. They hope the discovery will eventually lead to a vaccine for the Aids virus.

Human trials are some years away and experts say although this discovery is important, it is just one of a number of avenues being explored in the fight against the disease.

Rapid test means prompt treatment

Meanwhile, in the United States, government health officials say new rapid tests for the HIV virus could help thousands more cases of infection to be detected.

They say the tests now commonly used, while more precise, can take weeks to process and that many people who are tested never return to find out their results.

The Centres for Disease Control and Prevention estimate that 8,200 more people a year would learn they were HIV-positive if a commercially available 15-minute test was used at publicly funded sites.

Aids awareness campaigners believe this would mean more people have access to prompt education and treatment.

"Most people either don't want to or are afraid to deal with it unless they become sick," said Tony Braswell, executive director of AID Atlanta.

"If you can tell someone while they are sitting there, talking with a counsellor, you could get a head start with them and tell them that their life is not over."