The Flood
The Flood (called The Parasite by the Covenant) is a species of virulent, parasitic organisms that thrive by consuming other sentient life of sufficient biomass, no matter where in the universe, thus leading Forerunners to the creation of the Halo Array. The Flood appears to be a hive or communal mind, existing in several stages that eventually lead into the creation of a central intelligence known as the Gravemind. Next to nothing is known about their origin, except that they were created from something Precursor. Whether they were actual Precursor remains or created by the Precursors or both is unknown. The Flood settled upon a Forerunner world, where they were found and subsequently battled. It has been revealed that humans discovered and then fought them while fighting the Forerunners until they were defeated by the Forerunners, who had no idea about the Flood, and devolved humanity after centuries of war leading to their solution, which didn’t involve wiping the whole galaxy clean being lost. Faced with the Flood spreading throughout the galaxy and consuming every sentient being, the Forerunners created twelve artificial Rings called the Halos. The point of the Rings was to wipe out all sentient life, thus starving the Flood of any organisms of sufficient biomass to sustain them. Through their own interpretation of Forerunner artifacts, the Covenant believed these rings were actually devices that allowed the Forerunner to transcend this plane of existence into godlike state, and sought to do the same by activating them once more, though doing so would wipe out all sentient life in the galaxy. Although it seemed the plan had been successful at eliminating the Flood threat, samples of the organism kept on the rings for study were released on at least two of the installations through accidental means, putting the Galaxy once again at risk of complete destruction by the scourge.
The Flood grow and reproduce via the assimilation of other beings, using corpses and found biomass in order to spread. They are incapable of sexual reproduction, instead they fester inside a host until a certain time period, after which they explode and many new Flood are given off. Once a Gravemind is formed however, the Flood advance to a stage where they no longer require hosts for this, only biomass and calcium, which they require to feed on, and to build bodies with. Once they have advanced, they create hives which can produce thousands of Flood inside large pustules on walls and other surfaces, which act as incubators. During this stage, the Flood are able to build their own life forms using captured biomass and calcium deposits. The Flood continue in this fashion recurrently until either everything is devoured, or until they are stopped, the former usually occurring.
The Forerunners
Once the dominant civilization in the Milky Way galaxy, the Forerunners reached the pinnacle of technological advancement approximately 100,300 years before the Human-Covenant War. Apparently charged by the mysterious Precursors with protecting less advanced civilizations in the galaxy, they fought against the humans in a Human-Forerunner war. They then stumbled upon the Flood and fought a three-hundred-year long war to stop the parasitic species from consuming the galaxy. When it looked like they were going to fail, they built and activated the Halo Array - sterilizing the entire galaxy of sentient life including themselves, starving the Flood into submission, and repopulating the galaxy again with those few species saved from extinction.
All that remains now of the Forerunners are the ruins of their ancient temples and cities, a few scattered pieces of functional technology, and their automated combat and maintenance constructs.