There is not one scientific reason. What is known, however, is that with some people it's genetic, while, with others, it's a will to live. Each person is different and some people have a strong immune system that helps keep the HIV under control, and they become what is called a long-term non-progressor. Researchers are studying these people to find out how they're able to keep the effects of HIV under control. Also, the people who have lived longer have made specific choices - sometimes unconsciously, which have ensured a longer life. These choices are all about reasons to live and seeing purpose and pleasure in their future.