And as I said,
"Silentium Flood is essentially the catch all moniker used to describe the Flood once they achieved such critical mass towards the end of the Forerunner-Flood war (due to having consumed trillions of bodies, millions of ships and countless ancilla therein) that they formed complex command nodes encompassing whole star systems referred to as Keyminds. Once they achieved that stage their powers of parallel processing reached such a critical point that they could reactivate long dormant Precursor artifacts - exceeding the point in which the Flood is now spontaneously growing from raw resources as opposed to living organisms. Essentially the Flood we see in the games, but having consumed enough host bodies of two exceptionally advanced interstellar civilizations that it then begins to form an equally advanced gestalt self-replicating swarm intelligence. The term Silentium refers to the novel title Halo: Silentium which is set during the final years of the Forerunner-Flood war.
The key to understanding the threat posed by the Flood outside of their use of neural physics is purely intrinsic to their wholly exploitative nature. To quote Auntie Dot, the mathematics are determinate.
First, as I've said before in the past, the otherwise impossible incubation period of the FSC would normally prove easy to quarantine in most cases, however whereas most pathogens disable or kill their hosts the FSC mutates them into superhumans with an active psychic bond that naturally seeks to form a central, highly organized intelligence capable of effectively fighting and mobilizing against any military force it encounters. The sequencing system for geometric progression is calculated as (2x2x2x ... x2^n-1), with vectors expanding as a continuous variable. You turn into a Flood form and infect the next person, and the virus is carried over to two people who start the chain (2 = random model, but we’ll treat it as a baseline for now). The next minute, each one then infects two of their friends. Now four people are newly infected. Each of them infects two people the third minute, and eight people are infected, and so on. These events can be written as a geometric sequence with n-1 being the variable of time. So in geometric sequencing you don’t assume a consistent rate of attrition, instead the figure is treated as an integer with a constantly fluctuating value (the more Flood you have, the faster it can potentially spread through restrictive elements in prismatic fashion, thus the value of n-1 isn’t a technical constant, neither is the random model). As long as you have a common ratio the figure can progress as a baseline constant for the duration of growth as (a^2/a^1) = (a^3/a^2) = r. So the initial vector starts at two, but within the first hour that figure expands to 2.3 million, thanks to increased assimilation from Flood expansion and the Flood’s ability to manipulate technological craft and move faster than the original host (thus you have multiple vectors of transmission) that figure rockets to 8 million by the next hour, five minutes after that the continued expansion inflates until it reaches a stopping point (i.e. the population of a large city), but since you now have all manner of random outcroppings of the virus (thanks to starships and interstellar portals and teleporters) that same random variable is fluctuating wildly in nearby regions.
Secondly, the Flood are not merely a brute force homogenizing swarm, but a highly adaptive form of corruption and data distribution. Single individual forms are designed to act as a logjam of local information among coordinated military efforts through sheer numbers and utter fearless tenacity, the intent being to propagate and limit effective response to systematic population depletion. Meanwhile the Flood continue to form a gestalt FTL psychic network from which they extract the sum total of memories and operational processing of the native organics and their computer sysems. The issue isn't that there's merely more infected bodies to kill, the issue is that the more the Flood consume the more knowledge they extract from the native environment and thus the greater their processing capacity becomes as a result. You're not just fighting super tough as balls to kill Zombies with bullets, but a distributed organic intelligence network that now knows everything you do (from which button vents the atmosphere to how to pilot a ship) while in turn possessing the combined processing power of the sum mass of organic and synthetic material consumed. In the case of the Keyminds this processing exceeded the capabilities of even Contender-class ancilla It's a self propagating homogeneous swarm intelligence with an instantaneous OODA loop that increases in verisimilitude and processing power along a variable of n-1. And that's not even getting into the more esoteric stuff such as the Logic Plague.
Speaking of the Logic Plague, the Logic Plague is described so inconsistently that it probably represents the Forerunner parameters for a series of OCP events rather than a singular assault. First it comes from an esoteric subversion of AI runtimes outside of Forerunner data parameters (so much so that the AI itself cannot detect the intrusion) that leads to a stream of exchanged consciousness that subverts the already compliant intelligence. Then in one instance it's the Gravemind poking his virtual tendrils into an FTL communication relay and infecting a widespread data network operated by cyborgs, the next it's merely proximity contact with organic Flood forms (not even the Gravemind is involved) that rapidly subvert AI, finally it literally becomes a form of malediction - an arcane term in which even organics can be quietly tweaked for the long term propagation of the Gravemind's goal. The Logic Plague seems to be less of a definition for a lone computer virus/trojan horse/software intrusion/a conversation/data exchange and more of an engineered memetic instance through multiple higher vectors (data, ideas, direct dimensional manipulation etc.) intended to directly benefit the Flood and propagate to it's natural end state (i.e. total saturation), sort of the equivalent of a higher context, high volume smothering that's defined broadly by their inability to predict, defend against or even adequately expunge the corruption itself.
Thus it can be indeterminately logical, infectious, mechanical, digital and physiological - all at the same time. The most simple way to explain it as an overwhelming assault that cannot be countered through conventional means or even understood with known processes (Cortana also learned the hard way that attempting to engage the Flood in a pure computing war is pointless), but something that's best put away in a box and hope it never escapes."