HIV is a virus that enters lymphocytes within the bloodstream. They get in this position by either a direct inoculation into the bloodstream (perhaps from a blood transfusion or sharing needles with people who are doing intravenous drugs) but the most common event is inoculation into a mucosal area - either the rectum or the vaginal cervical tissue. They enter into cells that are in the mucosal areas and then trafficked into the bloodstream, entering millions and millions of lymphocytes. So most of the HIV in the world is transmitted sexually. This is why there is so much emphasis worldwide on reducing sexually transmitted diseases. Those that cause ulcerations on the genitalia are particularly conducive for transmitting HIV from one individual to another.