On the 27th of April, 5029 AD, the first death from an hitherto unknown disease occurred on Heronrest. Over the next thirty days, 90% of the population of Heronrest and the surrounding forty systems died. The 'Snow Plague' spread like wildfire throughout the worlds of the Human Raj, whose quarantines always seemed one step behind the plague. Within a year, 80% of the human population was dead. The Naram, who were willing to bomb cities to maintain quarantine, kept their casualty rate down to around 65%. Entire planets were wiped out, while some others, isolated and cut off, drifted into their own dark ages.
The KessRith leapt to the offensive, and over the next century, humanity was pushed back and held to Ten Core Worlds ten core worlds around Terra. The Naram managed to maintain over a dozen worlds, but at horrendous costs.
The cause of the Snow Plague is well known, but its source is still a mystery. It is actually a combination of the common cold virus (hence the name) and a trigger virus. The trigger viruses (of which there were six varieties) were quickly isolated, but where they came from is a mystery. Many point to obvious genetic tampering, implicating a KessRith or Ssoran (or perhaps even a Vauvusaran) origin, but no proof exists, and the parties involved all deny it vigorously.