What causes mould?

Mould is caused by tiny spores, which are present in the air continually, both indoors and outdoors. When they land somewhere damp, they begin to grow and digest whatever they are growing on to survive, especially dust and wood particles. When there is excessive moisture indoors, mould can flourish, and this sometimes goes undetected for a long time, if the mould is behind furniture, or in a secluded corner.