Mould spores (like “seeds”) are found, literally, everywhere, including our homes. In our homes there is amble amounts of organic material, “food” for mould, such as: wood, paper, cardboard, cotton, wool, linen, many, many things that are organic. “Dirt” contains particles of organic material and dirt is a common food source for many moulds. Many moulds thrive in the temperature range of 20C-30C, and our homes are typically kept inside of that range. The only thing missing for mould to thrive is “water!” If there is a leak or broken hose or pipe, or even high humidity (over 60%RH) then mould has enough moisture to begin to grow.