helpl with mildew growth on exterior of wood home

We have 100 year old Victorian outside Chicgao. In the past year, we have noticed a significant amount of mildew gorwth on all sides of the house (not just sunny or shady sides). The house was pinted three years ago, but there is no gurantee against mildew. They used a stain rather than oil based paint. It is even happening on the garage in back.
Two questions:
1)what is causing this and how do I stop it? 2)am I going to have to power wash the mildew off and repaint with a mildew agent in the paint?
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