The key genes which lead the body's fight against HIV infection have been pinpointed by scientists.
HIV can mutate rapidly
The work provides a greater understanding of how some people can survive symptom free for years, while others rapidly develop Aids.
Researchers hope it may also aid the development of a vaccine to prevent HIV infection.
The study, a joint project by the Universities of Oxford, KwaZulu-Natal and Harvard, is published in Nature.
Experts believe there is the equivalent of an evolutionary arms race occurring between HIV and the body's immune system.
The virus is able to mutate rapidly to side step the immune system defences.
This, in turn, is driving the rapid evolution of the immune system, as genetic variations which are less effective at combating infection are being wiped out, while more effective forms have a greater chance of survival.
Normally, the process of mutual adaptation would take thousands of years.
But the latest research suggests that in the case of HIV it is happening much more rapidly.
Not only are human immune responses to HIV causing rapid changes in the virus, HIV is also causing rapid evolutionary changes in humans.
The researchers focused on molecules - called human leucoctye antigens (HLA) - which tell the immune system which cells have been infected with HIV, and should be destroyed.
There are three types of one specific type of HLA, but it has been clear for some time that type B genes are evolving more rapidly than either type A or C.
The researchers collected data from HIV-positive women attending antenatal clinics in Durban, South Africa.
They found that type B molecules do the best job of identifying HIV infected cells for termination, and consequently the speed of progression in the infection seemed to be strongly linked to which version of the B molecule gene each woman carried.
Not only were HIV-positive women with a protective form of the gene more likely to survive, they were also less likely to pass the virus to their children.
However, it also seemed that the fastest changes seen in HIV are those driven by the need to combat B type molecules.
Lead researcher Dr Philip Goulder said: "We have known for some time that HLA-B molecules are evolving more rapidly than other types, but it has been unclear why this is happening.
"These data suggest an explanation for the more rapid evolution of HLA-B in response to other infectious diseases, not only HIV.
"This is an exciting time for infectious disease research because we are witnessing the evolutionary fight between the human immune system and the HIV virus happening right now, rather than over a period of thousands of years.
"This study identifies the genetic battleground where the struggle between HIV and the human immune response occurs.
"The findings will help in understanding precisely how the immune system can succeed or fail against HIV, a prerequisite for a rational approach towards design of an HIV vaccine."
Jo Robinson, a senior treatment specialist at the HIV charity the Terrence Higgins Trust, welcomed the study.
But she said: "We should stress that we are many years away from having a vaccine for HIV and from being able to translate research like this into more practical options for people with HIV to managing their health and treatment."