originally posted in:Halo Archive
The precursors were not the Flood. The precursors created the Forerunners as slaves. The Forerunners rebelled and assumed the mantle. The Flood are a bio weapon developed by the precursors. The Humans accidentally stumbled across it and it slowly evolved and infected their "dogs". It eventually became known as the shaping sickness by the humans and the flood by the Forerunners. The gravemind Mendicant spoke to was a precursor mixed with a variety of other species.
English
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The flood originally entered our galaxy in mysterious and very basic ships in the form of powder which humans used on their "dogs" and that is where the infection started. The ships came from a precourser star system in another galaxy. The librarian went to that source and found the abandoned forerunner fleet and forerunners living in exile in a once majestic and precourser occupied system