It was the 1150s-1200s before the europeans had enough food and heat to sustain large populations. Weather was warm and more people eating beef. Some of those claimed by the plague may have actually been anthrax (In The Wake of the Plague). As for the Romans 240-250 AD small pox
400-425 AD Gohnerrhea (sp).
750 AD The plague
It was the 1150s-1200s before the europeans had enough food and heat to sustain large populations.Around there I woulds imagine. In the Domesday Book the town that my family originated in had a population of 250, that was in 1068, in 1279 the population was about 1500 with about the same area.
I do think though that small pox was a symptom of decline reather than a cause.