Right, just a few flaws:
Space marines have airtight suits and are superhuman? Master chief and in fact any Spartan is superhuman through genetic and physical augmentation and are in spaceworthy suits.
The flood have been known to overcome any anti-infection device or defense. What makes you think a couple of squishy organs is going to help?
Necrons still have minds, and the gravemind can overtake these, sure not to the extent of other psychic being in 40k, but nonetheless possible.
Who said the flood needed humanlike nervous systems. What evidence is there that the elites, grunts, brutes, prophets etc have this type of system? Also iirc, the flood can take over / infect Hunters. Now hunters are a large mass of worms. All they need to infect is sufficient biomass.
Orcs are fungi eh? Well here's the thing, the flood kind of are too, they'll just absorb them into their genetic code. Though gravemind would allow a temporary allegiance with them because of the similarities. At least until which point it is sure it can infect a substantial enough portion of the orc hordes.
Insects? See above point about biomass. THEY INFECT EVERYTHING OF SUFFICIENT BIOMASS. If they can't at the time, they adapt and evolve.
Each flood is relatively unintelligent, but as a whole they are exceptionally smart and have capabilities on the level of mind control, teleportation, etc. Motor skills are an issue, but some of the more natural flood forms are very agile and don't even need external weaponry to attack (I'm speaking about the form that fire needles like a turret, but can also run around anywhere with great speed).
Graveminds are stronger than you think, and there are also many of them. After taking over orcs or tyranids there would be enough biomass to reveal much higher psionic powers in the graveminds.
And lastly there is a difference between too stupid and complete inability to stop: everyone would be too stupid to allow themselves to be taken over by the flood. The thing is, its damned near impossible to stop.
I Am Raven wrote:Math is like a penis: it can be very satisfactory, but also a pain in the ass.
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Caboose: Not my fault. Someone put a wall in my way.