This has probably already been asked a lot, but how much control does the Flood have on its infectiousness?
As we all know, breathing in spores is a way to get infected by the Flood. The most famous other way is being hijacked by an Infection Form. But wouldn't being slashed in the face by a Combat Form technically put some of its biomass in the wound? We see them constantly flaking and seemingly shedding parts of their biomass, so I think it's reasonable to assume that receiving an open wound would create at least a risk of infection, but I never found anything that explicitly states that it does. I guess since their "designation" is specifically combat, maybe the only mobile Flood form capable of direct infection are the aptly named Infection Forms, which would explain why the Combat Forms don't infect everything. I guess there's also Pure Forms, which I assume might be able to, but I don't know.
Furthermore, in Halo 2, the station where we first meet the Flood has us fight alongside Elites and Grunts. The Grunts obviously have their breathing masks so they are technically not breathing Flood spores. The Elites have their shield which I believe protects them also. But when their shields are down, shouldn't they basically instantly get infected?
Also, the Gravemind on Delta Halo willingly didn't infect the Chief or the Arbiter because it needed them. But this begs the question: does the "flesh" of a Flood infected being carry the supercells needed to infect, or are they a special secretion of some kind, which the Gravemind decided not to use? Because while the Arbiter has shields, he's being held directly by the Gravemind and still is breathing in the likely contaminated air around him. Since Rtas 'Vadum can still smell the Flood on the station, I assume some must get through the shield. Being in such close proximity and physical contact with the Gravemind should have, by all means, been detrimental, at least, to the Arbiter, no?
Can the Flood decide each time which potential hosts it takes and which it doesn't?
My understanding is that the Flood works as a sort of hivemind, where each bit of biomass responds to a Gravemind. If that's accurate, does it mean the Gravemind has dominion over EVERY aspect of the Flood, including supercells? I assume it does, since otherwise, an Infection Form could have hijacked the Chief or the Arbiter and that'd be that. The Gravemind is impossibly intelligent and complex, so I have no trouble imagining it could very well control each individual bit of the Flood, dictating which potential hosts do get taken and which ones don't.
But there's also a counter argument to this, since while they're technically allied, Master Chief ends up fighting the Flood again, so either each individual sentient form has its own will, or the Gravemind decided willingly to hinder the Chief's progress to stop the firing of Halo, which would be catastrophically counter intuitive for the Flood.
So, I guess my question is: what dictates who and what gets infected by the Flood, and how does it explain each and every case of the Flood having the power to infect something, but not doing it (either through the Gravemind itself not infecting the Chief and the Arbiter, encounters with Infection Forms or breathing spores)?
For context, I only ever played through CE and Halo 2, so if there's more context given in the other games or external media, that's my bad, but through the research I did, I found nothing conclusive, so either we don't know, or I suck at Google. Also sorry if this is a bit all over the place, it happens when I write a lot.