I try and keep my basement area reasonable for storage of the massive collection. I run a dehumidifier, try and quarantine things that may bring in bugs, and check the temp/humidity in the central collection space.
Problem was- I had a slow leak in a drain pipe so whenever I ran my kitchen water it dripped down where I could not see it easily. When it did this the humidity of the area went up, but was not seen on the gauge. So the area started to grow mildew.
And I guess it spread form there. So I am now rooting out pockets of the crud where the air had not circulated. Thankfully we have had some very sunny days as mildew is best killed with UV light. I am also running an air filter in the basement to suck all the mildew spores out of the air.
One place I found a pattern of it on the top layer inside a box where the cardboard flaps did not fully close. So the spore stuff must have settled there on an air breeze. Had I had the box sealed better I might have prevented it.
So I am converting more of my lesser stuff from cardboard into better plastic boxes. Fighting this horrific thing is a pain. Plus the spores are supposed to be nasty to people, and I have a cough now that will not stop.
People spend far too much time looking for stuff for their collection, and not enough taking care of what they have.