.No idea.Possibly the precursors introduced it. Possibly the precursors introduced it. To answer that question, you'll have to ask the Forerunners. No one else has any clue, even the Forerunners were a bit fuzzy on the subject themselves. All we know is that the Flood was in it's purest form (a cell-based liquid) when first discovered, so maybe it was dropped off by some kind of Flood seeding ship, or came here in an organically-based vessel that dissolved before the Forerunners arrived (They didn't know how long the Flood had been on that world anyways).