Mould and damp can be a real problem within buildings, especially older ones. It can be caused by a number of different factors.
Damp coming down from above will probably be one of the following: leaking roofs and guttering, cracks in render, leaking pipes.
Damp rising up from the ground level will probably be caused by: increase in ground water levels, damp proof course failure, blocked drains, incorrect installation of new structure around the building.
Mould can be a sign of dampness from these factors, or can be caused by other factors like: poor ventilation, excessive water use in area, compromised breathability in the building structure.
There are some environmentally friendly alternatives that you can use in order to tackle damp and mould, but if in doubt about where your damp or mould problem lies, it is best to consult with a specialist or reputable builder as you will need to deal with the underpinning cause rather than just 'papering over' the symptom.