the one true God is Doctor Doom and we should all be worshiping him.
Come to think of it, Xenomorphs have an adaptable biological and genetic structure, considering that they gain attributes associated with their hosts. As far was we know Xenomorphs don't need oxygen or even breath it as in the first movie Ripley ejected it into space and it neither depressurized or suffocated. It didn't even freeze.That could be attributed to movie effects at the time but considering ripley killed it with the firing of the engine we can only assume that otherwise, the xenomorph would have been fine if left un-toasted.Right there we have a creature that can't be infected through breathing in spores, has a tough exeoskeleton that might even contend with the needle of an infection form, and blood that will dissolve anything physically injected into it's system.Take into account that a xenomorph has an exoskeleton.Most life forms with an exoskeleton have no internal skeleton, so what does that mean?No bones, no calcium deposits.So theoretically, the flood would have to adapt a toxic form that produced toxins on par with a xenomorph's blood, breach the exoskeleton, and then infect it. But the lack of bones would dictate that the flood in an early feral stage couldn't infect a Xenomorph unless they already had plenty of calcium deposits and could create pure forms.And I'd wager that's not easy.