Undetectable means that the HIV virus is suppressed to the point that it is not detectable from a blood test, and you get that way from taking meds called "cocktails" or a combination of drugs used to fight HIV. Someone who is Undetectable can not transmit the virus, but even with that doctors still recommend that people who are Undetectable still should use condoms when having sex because of the other STD's that are out there, its because people with HIV have a weakened immune system and other STD's can cause problems. And remember just because someone has HIV they are still a human being