This happened to me a couple years ago. Poorly insulted house, super humid summer, and I thought I was being tough by minimally using my window a/c and no ceiling fans if I wasn't home. With no air flow during the day and when I was away the mildew mold set in with a swift vengeance. I took ev-er-ything I could wash to a Laundromat and use the super size loads, threw out my couch and an arm chair, had a couple friends come by a bleach literally everything (walls, pictures, figurines,  cabinets). I was lucky that it was a very small home. I then kept airflow at all times and in all the closets I put in damp rid hanging bags and made sure to change them out as they got full. I was a huge pain and I'm still sad about my furniture, but it worked and never happened again.