What do you think the new weapon(s)/enemie(s) are?

[Dole]

Brutes can't be infected because their fur is too thick for the flood to penetrate it.

If the Flood can penetrate the Chief's air-sealed space worthy armor, I'm sure they'd have no problem going through gorilla hair.
Even though in First Strike they described the Brutes as having very thick, tough hides, I have no doubt the Flood could pierce that, also.

Probably. I am sticking to the excuse that brutes don't have enough brain matter/intelligence to infect.

They can infect swamp plants but they can't utilize the mental capacity of self-aware beings capable of speech, social interaction, comprehending the concept of religion and spirituality, and the ability to learn how to proficiently operate and manufacture space-age equipment?