Environment - Biological Agents
Mold spores are always in the air. In order to grow, however, they need specific conditions. They like it warm (above 75 degrees °F), moist (above 60% RH), and an area that is dark, with little air circulation. A basement is a good nursery for mold - or the area behind a frame on a damp wall.
Insects, of course, are drawn to all sorts of organic materials.