Not running fans in the bathroom, running humidifiers, not properly cleaning and not venting dryers properly (as you learned).

Turns out the humidifier on the furnace was left on all winter so far and the windows and walls had condensation and mold.

Two big causes of moisture I have observed are tenants running humidifiers and not using exhaust fans when in the shower (make sure your bathroom has an exhaust fan).

My grandmother is one of these people to me, she buys essential oils, and quarter collections, humidifiers and dehumidifiers cause somehow having both running is better than having neither. 

Damage went through the floor and into the ceiling and walls of kitchen and laundry room below.June 7: Insurance company sends someone to inspect damage, set humidifiersJune 10: They come back to remove humidifiers June 14: Insurance submits claim and payment to meAt this point, the tenants father, who is in the business of plumbing and renovations, informs me that they seriously underpaid for the amount of damage. 

The oxygen tension is already lower yet you're wearing a mask which will impair your ability to clear your eustachian tubes, circulate and humidify the air in your nasal passages.

There is nothing growing on the walls or anything, but if you go in there and sit on the couch or the bed it feels cold and damp like it was in a refrigerator or something.I don't believe there are any leaks or anything, I pay for the water bill and the bill is very reasonable, ~100 bucks a month for all three units.There is a crawlspace on the other side of the wall of her unit where the water heaters for the units reside and there are various things stored like yard tools, is it possible that rain water is getting into there somehow and leaking under the floor, humidifying the room?

I run 1 Whirlpool humidifier as needed snd it does the job (best value I got online from Costco).

Tried using a humidifier in the home?

Patrick,  A humidifier would be good for after you close to help get the moisture under control, so would be just letting the house breathe with the windows open.