Like most creations, the Flood had originally been conceived by a brilliant - now extinct, absorbed - alien race before the time of the Forerunners for a wholly different purpose. But the Flood was unique. It was actually created in two parts, the first part originated with the Precursors, a race which had for all intents and purposes, created the Forerunners in the first place, and the second was actually created by a relatively primitive alien race, who were seeking to find a way of creating a parasite which would cure virtually all diseases and illnesses on their homeworld, and granted immunity.
Unfortunately, they didn't realise the long-term consequences of their creation.
And, even worse and more tragically, they didn't know it would hold terrible consequences for the galaxy at large.
It was originally designed as a parasite, which when introduced into the bodies would then seek out and absorb diseases, foreign viruses, and lethal bacteria. Once it had, then it would genetically rewrite the DNA, and transform it into something more benign.
Once everyone realised what they could have, they were excited.
There were so many dozens of applications for such a parasite, they excitedly realised; with this parasite, they would be able to make themselves not only immune from diseases and illness but could, with a little work, find ways of using genetic engineering work beyond their level.
Except when they introduced the Flood into the test subjects, choosing subjects afflicted with all kinds of illnesses, ranging from cancers, their form of leukaemia, and blood diseases, to diseases comparatively mundane like their version of the common cold virus which would cause so many problems on the future planet, Earth, the Flood treatment worked too well.
And then it all began to go wrong. The test subjects found the Flood parasites quickly going to work, absorbing their afflictions, and they quickly felt better…but then the Flood mutated. It absorbed them all, consuming their genetic codes and rebuilding them into shapeless creatures that lashed out at everything around them. Too late, the scientists who'd worked on the parasites discovered they wouldn't just absorb the hostile DNA, but it would absorb it all. They panicked and tried to shut it all down and destroy the parasites, but it was too late.
Worse, it was soon discovered that the creatures, far from being unintelligent, had absorbed the intelligence of the people they'd absorbed. Soon the creatures began absorbing and consuming scientists and doctors, growing in number and growing in intelligence. The whole world went to war against the parasites as they reproduced rapidly, like insects, absorbing animals, insects, and plants. It consumed their DNA, cherrypicking the best bits. The most terrifying thing about the parasites was they were constantly evolving into genetic predators as they slowly exterminated and consumed the unique life forms on the planet.
But the inhabitants of the increasingly overrun planet refused to give up. They used poisons, genetic weapons, and nuclear weapons. While they worked, the parasites absorbed a lot of DNA strands, and a lot of people were afflicted with illnesses and viruses. They also showed a cannibalistic ability, they would absorb their own dead, and they began to breed and become stronger. They began to breed.
And breed.
And breed.
And breed.
They continued to absorb many people, absorbing their DNA, absorbing their knowledge and they grew in knowledge in power. As on many worlds, this planet had oceans full of diverse life, ranging from mammals, reptiles, microscopic life forms, forms of coral and sea plants, to the parasites that were dinner. All life as they had once been became extinct without one of the planet's years, and soon the last sentients were absorbed. But the parasites were so hungry and as they absorbed each other, creating new strains like the diseases they had originally been designed to destroy and repurpose, they became obsessed with their own survival. They knew that without any more life on the planet to absorb, they would soon die out. Delving deep into their collective memory, the parasites 'remembered' space travel.
Quickly they began creating a series of generational ships, although the new combined intellects they had absorbed allowed them to experiment with engines that took them closer to the speed of light and they left the planet in a wave. One of those ships would eventually find the dust which had previously been the survivors of the intelligent race known as the Precursors, who had fought a long war with the Forerunners, who'd gone to war with them after they'd been passed over as the species who'd fill in the role of protectors of the galaxy.
The dust would be reconstituted, the Precursors knew, when enough time had passed and they would assume their original form. But the Dust became defective, centuries in. Soon, it began to absorb DNA and caused sicknesses and diseases with whatever came in its way. And then the parasites came to the planet. Only one, heavily mutated Precursor, managed to survive and sealed themselves in a stasis capsule with the knowledge of what the Flood was.
But the two breeds of genetic virus both found common cause and they became a massive galactic threat once they'd absorbed each other, and they went into the galaxy. After consuming many worlds they came across the others of their kind and consumed them, and the planets and races they had absorbed and finished off added to their DNA.
The Flood virtually became unstoppable, and, as if guided by a deep-seated desire for revenge from the Precursors, the parasitic swarm targeted the Forerunners. They overran many of the Forerunner's worlds, absorbed hundreds of them….before the Forerunners, who were running out of their options, came up with the daring, deadly, last-ditch effort to destroy the Flood by devising the Haloes to deliver a terrific blast of power throughout the galaxy. They had collected DNA, living specimens, and embryos from hundreds of worlds from all across the galaxy, and after they had delivered the blast from the Halo network, the Forerunners went to work to restore life.
But they always knew there was a chance something had survived…