It has been recognized worldwide that the dreaded HIV infection takes place mostly under homo or hetero sexual contacts. There is still significant debate about the details of how HIV eventually overwhelms the immune system. One possibility is suggested by the antigenic diversity threshold hypothesis⁴. This hypothesis suggests that HIV's rapid mutation rate allows the virus to churn out a constant stream of escape mutants. As more and more escape mutant strains appear, the immune system has to keep increasing the number of strain-specific antibody classes at the same time that its being systematically attacked by the virus. Eventually the virus builds up enough diversity. So that the immune system may find it harder and harder to mount responses against these newly emerging virus variants. Once the antigenic diversity of the virus population has increased above a threshold value, the immune system can no longer control of virus, which results in the development of AIDS. This paper aims at developing a mathematical model to find the time to get AIDS after the first infection, is made taking in to the consideration of antigenic diversity threshold.