Each time you take a shower or a hot bath, the air in your bathroom warms up several degrees and its humidity approaches 100% — excellent conditions for growing things. You can see the effect of humidity in the fogged mirrors and the damp feel in the room. To help with this, run your bathroom vent fan for 30 to 45 minutes after bathing with the bathroom door pulled almost closed to keep the damp air in the bathroom until it is exhausted by the vent fan but not completely shut to provide a path for cool dry air drawn from the rest of the house. This will help clear the mirror too. You can get an inexpensive humidity gauge from the hardware store to track the change from high back to normal humidity to know how long to run your fan. If you have screw base vanity lights, going back to incandescents in the bathroom will help too — leave them on with the fan. If you’ve been fighting mold in the bathroom, this will improve the situation.