I'd have thought they'd still need a brain to breathe, eat and move. Wasn't it shown in Halo: The Flood that Infection forms merely hijack the brain?

Besides, in Halo 3, if you shoot a Flood in the head then it counts as a critical shot.

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"This one has forgotten whether it's heatsink is over capacity. It wonders whether the criminal scum considers itself fortunate" ~ Blasto, the only Hanar Spectre.