Mold intrusion into a residence is due to excessive humidity in the interior. Condensation forms when warm, moist air cools (by contact with the cold window), and can no longer hold as much moisture; just like a cold beverage container.
As Temperature decreases, the moisture content of the air is constant, but the ability to hold moisture is diminished. The 'overflow' results in condensation. This principle was the underlying cause of the water intrusion into the apartment, directly leading to severe mold infestation and health problems that followed.