When you walk into a room and experience that musty odour? You are actually taking in organic compounds that suggests mould is active somewhere. The issue remains with locating the root cause.
Moulds are simple microscopic organisms present virtually anywhere both indoors and outdoors and, to grow and reproduce they need a food source. Such sources can be wallpaper, plasterboard, paint, some carpets, adhesives, wood fabrics and other organic material.
Of note, mould in bathrooms is not feeding on the tiles but the biofilm residue on it and, because they are so small, a thin layer of dust or debris will yield a sufficient and sustainable food source.
In addition, and of equal importance, mould growth affecting contents can only be sustained by the atmospheric moisture conditions in the property. This being elevated atmospheric moisture levels and cooler air and surface temperatures.
Once established, mould in a property will quickly colonize areas with appropriate conditions and, although properties have periods of high occupational moisture, it is the prolonged periods which will allow mould to develop. When mould is present in a structure the period of time it will take to colonize new areas is reduced due to the increased level of spores being produced in an enclosed area.

The effect on humans varies greatly and can be simply understood by how quickly we each would get burnt in the sun? However, those in mouldy environments may suffer colds, flu like symptoms, viruses etc as a result. But, it is recognised that any ailment is difficult to prove and, the effect on humans can also depend upon on the amount of mould present, the type, the pathway to a person, how long they have been exposed and how sensitive the person or persons are.
No mould is welcome in a dwelling and its subsequent removal is key in order to remove the ongoing sporulation that will be occurring. Using a bleach product does NOT work as, although the surface will appear clean and clear, the root system of the outbreak will remain and, when conditions prevail for a reduced period, it will readily develop again.