Mould is in the family of “fungus” and is found wherever there is organic material, the right temperature range and moisture. In nature mould plays a key role in the decomposition of organic material. mould actually digests cellulose which makes up much of the mass of organic material. However, when the mould is inside structures it poses a danger to the structure, itself (wood framing, etc) and to the health and well-being of all occupants in that structure. (Note: mould also needs oxygen but, since that is found in air, i.e. “everywhere,” it is seldom listed as a requirement.)