We get mould indoors for a number of reasons, the following are some of the reasons.
Cracked tiles in the roof causing a water leak coming down into the ceiling and walls which you cant see but is causing damage from behind the walls and ceiling.
Having your furniture too close to the walls hence there is no airflow thus breading mould.
Mould can often be found in bathrooms that have no proper ventilation, this along with the constant steam from bath and shower causes mould to breed. This also applies to other rooms that aren’t properly ventilated.
Mould can also come from rising damp. You may have damp and moisture coming up from underneath your home because it is not properly ventilated and moisture is getting drawn up and into your home.
Leaky pipes in bathroom, kitchen or laundry walls which you cant see, but are slowly dripping away and over the years causes timber to rot and mould to form.