A closed-up vacation house can be a breeding ground for mold in the summer months. Moisture from a nearby lake or river, or the humidity in the air, can lead to that musty odor vacationers have come to expect upon arriving at their weekend getaway.
Mold is a particularly stealthy foe. It exists nearly everywhere in an inactive state, and all it needs to grow is a food source (drywall will do nicely), and a source of moisture, such as high humidity.