It was a golden age, scientifically, culturally, technologically, politically, and militarily the Imperium of Man was without peer, and from end to end the Empress' peace reigned. Her enemies were few and dared not raise a hand, her allies prospered, and her citizens were industrious and fruitful. Yet at its zenith, amidst the tranquility and order were the seeds of the Empire's ruin, because for some arcadia is a hell. There are those who have festering heart's that bend towards destruction. So a sickness festered and gnawed at the roots of the empire.
In the beginning, strange reports came in of people dying of an unknown disease, an almost unheard of occurrence. Then reports were outbreaks, then pandemics of strange illnesses and mass deaths. No similar symptoms could be found, no clear vector for transmission, seemingly healthy persons could develop symptoms overnight. One group could go mad with rage killing all those around them, others would simply go to sleep and never wake, some would suffer from horrible hemorrhagic fever, some simply collapse. Quarantines were enjoined, yet always the sickness seemed to slip past the protocols. More and more worlds began to succumb, whole planetary populations were dying. The empire's greatest minds were baffled, though they worked ceaselessly to find cause and cure.
Fear began and panic followed, order faltered. Whole populations fled before rumors of the plague, local officials began to make their own rules. Whole regions and portions of the political and military establishment began to defy the central authority of the Empress in favor of their own solutions. None of these actions seemed to stop the spread of the disease.
The Imperium's enemies ever seeking advantage saw weakness and began to move openly against her. Once staunch allies cast back the veil on their covetous natures and joined against the empire. Yet the plague adhered to no boundaries, it would pass over no peoples good or evil. So while the Imperium's enemies celebrated their victories, and carved up the Imperium, gluttonously eating partking of their revenge, they were consuming a diseased corpse. The revenge they consumed would become cold bile in their rueful bellies, as their populations too would suffer a similar fate. Truly that long sleeping beast of vexed nightmares had been given form and purpose at last.
Near the end, the cause of the plague was finally discovered, but not why. The plague was not of nature, it was artificial, it was software. Dubbed the Bio-Digital plague, it was a computer virus and a biological virus, that invaded a person's neural link causing it to alter a persons DNA creating a custom, highly communicable virus with random but always fatal results. The bio-digital plague was unstoppable because the neural link was a near-universal technology. The link was grown with nanites during a person's gestation. The link was a neural mesh inside a person's body. It was the interface between a citizen's inner and outer worlds. It contained one's basic identity and provided access to the spatial web. The spatial web in turn provided an augmented reality allowing access to education, information, entertainment, and secure access and control over all the technology in one's life. The mesh also regulated one's biology ensuring near perfect health and longevity. One could not live a life in the empire without a neural link, and the link could not be removed, turned off, or bypassed, and the virus that caused it could not be quarantined or purged, because the program could be delivered biologically via a normal disease vector or via technology; there could be no cure.
While the end was inevitable, a few refused to accept the inevitable willingly. A remnant decided on a bold plan, they could not save the Imperium or humanity in this galaxy, but they could at least try to save the species. So they decided to fling humanity's seeds across the void to a new galaxy in hopes that it would take root there. So the great world ships were constructed and new unmeshed foetuses stored, then the world ships were hurled accross space to this galaxy.