Mold is a particularly unsavory character that is neither plant nor animal. It usually resides in the fungi family and creates enzymes for decomposing and digesting and spores that are used for reproduction. In general mold and the decay they cause is not a bad thing; consider a world without decay. We would be piled high with dead trees, plants, animals and other dead organisms. Mold eats away at dead plants and animals causing them to eventually disappear. Unfortunately, though, damaging black mold doesn't know where to stop. It can't always differentiate between the things we want it to decay and the things we'd rather it didn't and this includes our home. Damaging black mold can also have a serious detrimental effect on our health if it is left unchecked.
Damaging mold thrives on moisture and prefers darkened corners because of this. In a house this can include many unseen areas that you wouldn't normally think to check. Behind washing machines, under the sink, in the attic, even behind the wallpaper and in the walls is where damaging black mold really likes to hide. Most of these places are rich with the moisture it requires and are infrequently, or never, cleaned. This means it can grow and grow but of course while it's growing it's also eating away at the very structure of the house in which it lives.
Houses that suffer from poor waterproofing are especially at risk because they have more damp areas and increased levels of moisture all around the building. By not waterproofing your home you are encouraging damaging black mold to grow wherever it can find a drop of moisture. If necessary it can suck the moisture out of your walls leaving the structure of your home in tatters. Of course, poor waterproofing isn't the only way that mold is produced in your home. A particularly humid house will have collections of condensation in many different places, regular condensation won't simply sit on your windows and drop off over time, it will collect on wallpaper getting soaked into the wall behind and it will collect just about anywhere it wants. Eventually damaging black mold is likely to grow where this moisture is causing the damage we know it can.
There are a number of ways to ensure that damaging black mold isn't attacking your house. Make sure that all air conditioning ducts are securely sealed, and installing a bathroom fan are two of the more common and effective methods of protecting your home from damaging black mold but there are many others. Ensuring you use permeable wallpaper backings and store firewood and other naturally moist substances outdoors. If you can smell a musky smell in some of the more prone areas in your house then there is a chance that the damaging black mold has manifested itself in your house. You need to hire a professional who will visit your house and do a thorough check of the entire structure. They will also be able to effectively remove it for you and give you some advice on how to prevent it from returning.