No One Can See Me, Hear Me, Feel Me, Or Smell Me. Yet, If You Tell Me

Riddle: No one can see me, hear me, feel me, or smell me. Yet, if you tell me... I can do all those things and more. Every person, animal, amphibian (or more) needs me. Without me my wielder will die. I can only be touched when my bearer is dead, and I can only be used if my bearer lives. You can create me cooperatively, but cannot create me manually. What am I?