When I was delivering tree work flyers around vacation homes, lots of places don't get used much, and high humidity in the woods. I could sometimes smell the mold coming from inside while putting the paper in the mailbox next to the front door. I wouldn't imagine it very habitable inside. On the other hand, a couple fans that bring in air from the outside on timers, work wonders, maybe two hours running per day. I think there is a service that will come in and open windows for some hours, if people are willing to pay. Most aren't, then they don't want to come because of it. The mold gets into the bedding...everything. Something called Black mold is supposed to be dangerous.