When HIV enters the human body, it begins an arms race with the immune system which is constantly attempting to develop new ways to destroy the invader.
http://www.redorbit.com/news/health/1112817108/hiv-vaccine-from-evolving-antibodies-040513/
http://www.redorbit.com/news/health/1112817108/hiv-vaccine-from-evolving-antibodies-040513/
