- last post: 01.01.0001 12:00 AM PDT
Some specimens of the Flood were kept in containment on the Halos. That's why there are still Flood, even after the Halos were fired. All of the "loose" Flood were killed off, but the ones in containment were set free (we think by the Covenant, though other sources lead to someone else entirely), reproduced, and created the outbreak that is occuring the games.