"At the end of this day one shall stand, one shall fall"
So, Death or Bungie?
Now, there are three theories on how the Flood was made and made it into the Milky Way Galaxy.
1. The Timeless One claims to be the last Precurser, it is unknown if this is true or not, but if Precursers are the Flood the... *shivers* Also the Precursers are a group of species, much like the Covenant,
2. The Flood was always a living creature and came to our galaxy because it had just finished destroying our closest galaxy.
3. The Flood was made by the Precursers as a last resort in case they were driven to extinction. When the Precursers were nearly extinct and fled the galaxy they released their last resort as revenge. This last resort was "Flood" but it was a powder at the time. After Humans and San Shyuum (prophets) got infected by the powder it spread quickly. The Humans later found a "cure" and the remaining flood fled the galaxy not to return for some time. When it did return the Humans had been reverted to Earth and tier 1, the Forerunners had no Allies.
So why does The Flood exist?
Take a look at this, The Timeless One explains how the Flood came to be and what its purpose is.
... the Primordial claimed that the Flood and the Precursors are synonymous. The Primordial provided further clarification on the subject of the Mantle, the purpose of the Flood, and the role of the humans in its ancient plan: the Forerunners were never meant to inherit the Mantle, and that humanity would succeed them in order to be tested for inheriting the Mantle.
It also implied that there is no true cure for the Flood, but that it could choose whether or not a victim was infected. Once this conversation was complete, the Didact, enraged about the revelation, fully activated the reverse stasis chamber.
Now for those of you that don't know what the Mantle is, it is way of life the Forerunners believed that they inherited. It basically states that they were to carry on life in the Galaxy. The Precursers passed the Mantle down to the Forerunners, then the Forerunners to Humans. This is stated false by The Timeless One shown above, he states that The Mantle was to be passed down to Humans not the Forerunners. Then he states that The Flood is a natural way of knowing that the species chosen to carry on the legacy of the mantle can handle it. To find out they use Flood spores, these Flood spores probe you and if that species is deemed not worthy by the spore, the spore mutates it. Hint hint forerunners, hint hint Johnson.
Tell me your opinions on all of this.