....Actually not a bad theory. We don't know when the Vex found the Black Heart exactly (and since they travel though time it doesn't seem to matter) and the Darkness is made up of the negative aspects of the Universe. One of which would have been corruption. So who's to say that during it's fight with the Darkness, in the Darkness's own last ditch attempt to kill the Traveler, it infected it to keep it asleep while it bought time to collect itself to come back and finish the job. The Vex with their ability to travel through time probably came from the past because it would have been easier to being the long building process then. (Time being fluid and pointless to them since they can lock things out of time) and with the Traveler out cold, it can't stop the infection of both the Vex and the miasma. Guardians act as the white blood cells to the Traveler clearing out the infection and letting the Traveler heal.