A species of monkey resistant to their equivalent of HIV could help scientists develop new approaches to human treatments.
HIV attacks human immune cells
Bizarrely, it could lead to therapies which turn off the immune response to HIV rather than try to boost it.
Sooty mangabey monkeys do not become ill when infected, but until now researchers did not have much idea why.
However, a team of experts from the University of Texas Southwestern Medical Center in Dallas, and Emory University believe they have found two key differences in the way that the monkeys' immune systems deal with the virus.
The monkeys appear able to generate only a low-level immune system response when confronted with an infection of simian immunodeficiency virus (SIV).
One potential treatment might be an approach to deactivate the immune system, in a very strategic and careful way
Dr Donald Sodora, University of Texas SouthWestern Medical Center
They can also keep their ability to make T cells - the kind of immune cells depleted in humans with HIV.
Dr Donald Sodora, one of the lead researchers, said: "The mangabeys have just as much virus in their system as during pathogenic HIV infection of humans.
"The riddle is, they don't get sick.
"The absence of the indirect effects in the SIV-infected mangabeys can at least be partially attributed to a reduced activation of the immune system, and the ability to maintain continued renewal of T cells.
"One potential treatment might be an approach to deactivate the immune system, in a very strategic and careful way."
This study was published on the website www.immunity.com.
Another study published on Monday also adds weight to the theory that the body's normal immune response actually indirectly assists the Aids virus rather than hurting it.
A team of researchers from the University of California at San Diego found that humans infected with HIV developed a strong immune response to the virus using antibodies - proteins which "tag" invading viruses and bacteria and mark them out for destruction by immune system cells.
The San Diego researchers found that while the antibodies were effective at marking out certain viruses, some were surviving.
This had the undesirable effect of promoting more durable strains.
Dr Douglas Richman, who led the study, said: "The neutralising antibodies are exerting a very strong selective pressure on the virus, and the virus is continually mutating to avoid it.
"The bad news is that the virus is always one step ahead, and the neutralising antibody response can't control it."
Richman hopes that the antibody response can be modified to cover a wider range of HIV, and be more effective.