I did a bit of online researching today, and it seems that dust build up around the windows/window sills gives the mould more surface area and nutrients to grow on.
So.....weekend jobs: buy mould cleaning/retardant spray and clean all around every window sill.
It's *much worse* this year - we've been in our apartment for 12+ years.... I think it's also because the last 2 years we were basically home, and would air out the apartment a lot through winter, whereas this year we're out and much more busy and are not taking as much care at home to do the airing and cleaning needed...
Clarification! What was implied by a gust of wind had nothing to do with fear of infections. Basically! The windows are so airtight that when the stove vent is on an under pressure is created. Thus when a window is opened the difference in pressure will fling the window wide open if not held back and the outside air will come gushing in.
Enjoy! But you are so right. Mould problems become huge issues if not tackled early and left to fester. Then it turns into a very unhealthy and difficult to solve nightmare.