Mold and mildew grow in high humidity stale air environments. Check the carpets, consider having them professionally cleaned, and do everything you can to assure there is adequate ventilation in the home. Make sure the bath fans run after showers, that windows are open allowing air circulation, etc.
Replace the furnace filters, if you have them. You mention the house is built on a slab. It is possible that the water leaches through the slab and just slowly enters the room, keeping the floor just wet enough to promote the mildew.
I'm sorry, but I don't remember the fix for that, other than striping the flooring altogether and applying concrete sealer to keep out the moisture.
Some health departments will do a consultation, you might take advantage of that...