Mould must always have a food source and a source of moisture in order to grow. The food source will need to be organic matter of some sort. In a building, this can be as simple as items like cardboard boxes, drywall with paper backing, plywood, etc. The source of moisture can be from a flood, a leak, or even excess humidity. Mould spores are always present in the air. If both the food source and sufficient moisture are available, mould is likely to grow.