The weather in the north east has been cool and wet all summer. The air conditioner which normally removes excessive moisture hasn't been on much because of the cool temps. I found some mildew in a corner of the basement for first time in 20 years because of the weather this summer in south west PA.
You could buy a dehumidifier to run during the times of the year with exessive moisture (spring and fall, and summer this year). That's what I plan to do.
Also, once you clean the areas with bleach and are ready to repaint. Use a paint with mold inhibitor designed for bathrooms, or add extra mold inhibitor to your paint.
I also recommend looking for any water coming into the house as suggested.
Remember that once you have mildew in a location it will return if there is moisture present for it to grow. Simply painting over it just provides a food source as it will eat through the paint in less than a year.