There was a great divide. Those who greatly opposed 'tainted blood' headed north, forming their own villages, religion, and moral codes. The winters were harsh, and the second winters were harsher. Yet, through their strength and their belief in the worthy, they survived.
Those who were tainted, or refused to follow the northerners, stayed. They created cities more like kingdoms, and a court to mitigate war and peace. They thrived as a population. They were complacent until another disease ravaged the land, killing hundreds of thousands.
Those who survived were often both lucky and unlucky. Many who survived were left blinded, handicapped, or sterile. The population only a fragment of what it once was. They knew it was because of their mixed blood, and felt the heavy weight of their ancestors sins. They were crumbling under pressure.
The tainted had nowhere else to turn but the northerners, who were unaffected by this epidemic. But being true to their beliefs, the northerners refused to taint their blood lines. They turned their backs on their mixed blood counterparts, not believing they deserved to live.
The northerners underestimated the desperation of the tainted. They underestimated what a species would do to stay alive.