For some reason I entirely forgot about the flood. That's rather impressive of myself.
Possibly because I never saw them as anything but "Hey, side thing to break up monotony". Then again, I've yet to play Halo 3 singleplayer for any real length of time. I should get on that.
Besides, let's do our defence on differences, aside from the fact there's 3:
Halo:
Humans: Hi, we're... in space because it's the future.
Flood: We're parasites that.... umm, parasite. Space parasites. Kinda like those real ones, but a bit more deadly.
Covernant: We're the aliens that aren't really invading till game 2. But we're in Space, humans are in space, it's war time!
Starcraft:
Terrans: We're criminals in space because Australia was full.
Zerg: We're created organisms attempting evolution by assimilation. Hey, you look yummy. What traits can you pass on?
Protoss: We're created organisms attempting evolution by physical and psychic power.
Xel'Naga: Yeah, you can blame us for fucking with creation. We don't show up aside from brief hints and maybe, just maybe we'll be in Starcraft 2. Till next time!