Falkreach isn't confirmed as the source, it's just the first 'civilised' place that it was spotted. The infected haven't degraded much over seven weeks because [s]I'm Batman[/s] magic. That said, you're right to say that the further North you go the less there will be and the weaker they will be too; but at this point players will have to contend with the environment and [i]other threats[/i] >:D