From Halopedia, the Halo wiki
Although the initiation of the Interstellar Stage has less to do with the Flood's biological development and more with their access to viable space travel, in the latter phases of this stage the full extent of the Flood's power becomes truly evident, transcending beyond its purely biological aspects. In line with the parasite's strategy of utterly saturating all available channels of information, non-biological intelligences and data networks are targeted with advanced forms of the logic plague, the Flood's information equivalent. With the assimilation of entire planets and the development of planet-engulfing Gravemind forms—the sheer intelligence and processing power of the Flood's collective consciousness enables them to manipulate neural physics. In the final years of the Forerunner-Flood war, the Flood was able to control Precursor artifacts through this effect, which made them nearly unstoppable against the Forerunners. However, all Precursor artifacts in the galaxy were supposedly destroyed when the Halo Array was fired at the conclusion of the war, denying the Flood access to Precursor technology in later times.
This is the stage the Flood was in during the final years of the Forerunner-Flood war and the events of the Battle of Installation 00. During the Interstellar Stage, separate Flood infestations across the galaxy will eventually coalesce into one singular collective which will swarm across the entire galaxy in a spiral pattern, moving toward the galactic center in order to ensure no worlds remain uninfected. Once there is no more uninfected biomass left in the galaxy, the Flood is theorized to enter the Transgalactic Stage.
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