From Halopedia, the Halo wiki
A Flood outbreak is a sudden increase in the spread of the Flood superorganism in a given span of time. Depending on context, outbreaks of the Flood may be localised to a small region, or may encompass large swaths of space.
Flood outbreaks typically go through a number of stages, progressing from the Flood behaving as mindless zombie-like creatures to incredibly intelligent organisms with the ability to operate starships and other advanced technology.
The feral stage is the first phase of a Flood outbreak. In this state, the Flood operates in a very aggressive manner, and their expansion depends on finding sentient life forms and expanding their overall biomass. Victims become hosts to the infection forms, and are mutated into combat forms. During this period of time, the Flood attacks with little to no strategy, seeming to focus primarily on consumption of intelligent beings and biomass and thus expanding and strengthening their own forces.
The Feral Stage is typically initiated either by an outbreak of extant infection forms, or the infection of various sentient and non-sentient species by Flood spores and other microbiological vectors. This infection soon becomes parasitic and causes the growth of Flood biomass in the hosts as well the generation of new infection forms to further the infestation. At some point during the Feral Stage, the Flood begin to collectively shift their objective. When this happens, Combat Forms will work together to accumulate the biomass of both infected victims and uninfected sentient beings, allowing a series of widespread chemical reactions to occur across the reserves of calcium and flesh they have collected, eventually creating what is known as a Proto-Gravemind. Once a Proto-Gravemind has absorbed enough biomass and becomes a full grown Gravemind, the infection can then proceed into the Coordinated Stage.
The coordinated stage is the second stage of a Flood infestation, over the course of which a local ecosystem is typically fully absorbed. During this stage, Flood population rapidly increases. The Flood Super Cell generates a hive and propagates spores into the local atmosphere in massive numbers. Once sufficient biomass has been gathered, the Flood gains the ability to spontaneously generate Flood Super Cells, removing its dependency on hosts and giving rise to pure forms. On the verge of initiating the Coordinated Stage, the proto-Gravemind continues to be built until it reaches the status of a complete Compound Mind — a Gravemind. It is at this point that the Flood becomes significantly more organized (communicating via thoughts and attacking strategically) and subsequently more lethal than in the preceding Feral Stage. Upon the formation of a full Gravemind, all locally available sentient beings have typically been absorbed by the Flood.
Once it has gained sufficient technology, the Flood will proceed into the Interstellar Stage.
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.
The transgalactic stage is the fourth and final known stage of a Flood outbreak. This stage is reached once the Flood has consumed all sentient life in an entire galaxy and gained complete understanding of any technological and scientific knowledge that was present. By this time, it has reached a complete critical mass, which means it must depart to other galaxies for further reproduction, infestations and new technologies.
During the human-Forerunner wars, the Flood was driven outside of the galaxy. Doing this may have caused them to achieve the Transgalactic Stage as they were able to return to the Milky Way 10,000 years later when they made first contact with the Forerunners on Seaward, sparking the Forerunner-Flood war. This is likely, as prior to the Halo Array's firing, Forthencho's essence was sent down to Earth by the Gravemind to tell the Librarian what he was told by it. Among this information was that the primary Gravemind had consumed at least ten thousand planets and many galaxies, reinforcing the possibility that the Flood has at one point in history reached this stage.
Several outbreaks of the Flood have occurred throughout history - some of the most notable are listed here. For more outbreaks, see here.