A Forerunner-based research team called the Primary Pioneer Group made active landfall on the bordering town planet known as G617g and they initially reported some interesting findings. Despite previous assumptions and conjectures, the world could support sentient life despite the lack of fauna, which they intended to review more closely. The protocol allows them to dispatch a secondary batch of researchers called the Advance Survey Team-Alpha. After a week of no communication, the Forerunners who were remotely monitoring this new discovery became worried. It may be assumed that the research team became the first (known) victims of the Flood, and either by their own transport, or by a relief force later sent, the Flood escaped the planet and began the largest genocide in all recorded history.
In their response to the initial testimony and report, those monitoring it advised that they would send a military detachment to the site shortly to uncover why their two exploratory teams had halted communication. That was the last time the Forerunners had any contact with G617g, and it was the beginning of the largest war their galaxy would ever see, lasting about three centuries. No one knows why the Flood were dormant on G617g or even how long they had been there, but it is believed that due to their vastly different physiology, they traveled to the Milky Way galaxy from another galaxy.