Sitting Water Causes Mold
This microbial growth in a popular store in Philadelphia, PA was the result of a water damage left to sit. When a water damage occurs, it is imperative to extract all standing water, otherwise mold growth is a very real possibility.
If you look in the upper left-hand corner of the "before" photo, you'll notice how the water seeped out into the ceiling causing extensive damage to the materials.
The after picture was taken once the mold had been removed and the area had been contained. Following the mold remediation, it is recommended an Industrial Hygienist come out and test the air quality to ensure the spore count is acceptable.