Since they both are a parasite they could become something horrid combined lol.
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Flood could infect the necromorphs, as long as living versions have a nervous system, or if they're dead (flood infect nervous system of living hosts, and force them to suicide (normally, but not always. Not the case in halo ce) then reanimate the body). Plus the floods innards are mostly fluid, only tissue in there is the infection form. So I don't think necromorphs can infect the flood.