When we were first married I ran a humidifier in our bedroom because I'd been brought up with the idea that a humidifier can help prevent colds. I kept the unit clean but when I took down the window blinds to paint I found lots of mold. All that moisture.
If I had to use a humidifier for a medical reason while sleeping, I'd be sure to run lots of fans during the day to air the room out and check all the nooks and crannies very carefully for mold. And clean the humidifier, itself, every single day.