Nugget: Human immune system indirectly assist Aids rather than attacking them.
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.