The virus may not have implanted the mutation in the other areas. It would be a rather rare event. But there is substantial evidence that shows that viri have caused several mutations in humans.
Better to say, the viri killed off more of those without the gene, but those with it lived to produce offspring they passed this to. I don't think that people normally acquire a mutation except at conception.
The virus could have existed all along in the forests of Europe. At this time, deforestation was going on all over Europe. In England there was a virus that would kill in single day that made the rounds as well... it was called "sweating sickness" and even killed an English royal.
Just as there are now sporadic outbreaks of Ebola in Africa... usually encountered by hunters in the bush, such outbreaks could have been common in the days when Europe had more remote forest lands and more people came into contact with pristine areas.