LB12Diamond said:
Was the parasite plague in the game.
Yes. They briefly referred to it as the Flood.
In the original game it went from a straight forward alien shoot em up game to horror all of the sudden when you find that these zombie like parasites are on the Halo.
I'll nerd out a bit. This is memory from 20+ years ago.
The Covenant think that the Halo is a sacred weapon to their religion to defeat humans.
The humans get to Halo right as the aliens do. Humans have no idea what it is but know they need to stop the covenant.
The humans find the "weapon" first. When they find it, they accidentally release the Flood which is the parasite you saw in the lab. The weapon is actually the Halo itself which is meant to not just wipe out the flood, but the Flood's food source (humans aliens, anything.). You go from killing aliens to fighting aliens and zombies (humans that have been infected )in a dark cave. And then you find that your weapons don't work on the zombies and have to change strategy mid game.
So the covenant still wants to use the Halo because they don't know what it will do. MC's mission becomes contain the Flood AND stop the Covenant from mistakenly vaporizing all sentient life.