are normally life-bringers, giving birth etc. .

Oh yes, I think that also factors into it. All in all she is a powerful symbol with plenty of meanings.
There is also the added fact that in Norwegian Plague is a feminine noun - which is rather unusual. No idea what came first though - the noun or the legend.

And the icons are gorgeous,aren't they? I'm so impressed with how the feel of the images have been transferred to icon-size.