I want know about the causes of HIV. Many persons tell about that , its transmission process can take place by sexually, or by making tattoos when the artist using a same needle, and other common reasons like else, I know these all are basic reasons which everyone know, I want to know how can HIV can take place by sexually, when no one has HIV from both who are making relation by sexually or we can say they are doing sex, My question is that, when there no one has HIV, how the HIV can take place, by sexually, when a person doing sex with a girl or one or more girls, where nobody there HIV?