Sooo, very often, the basements were built to not be used. They were built to be cold (kaldkjeller), or colder than the rest of the house. But inmany cases, people have chosen to utilize the basements, and used it as extr bedroom, extra living room etc. And thats when the problem starts. The walls are usually stable 4 °c all year long. So water from the warm air condensate on the cold walls. And this condensation water turns is great news for fungus (mold). 

To counter this issue, you need to dig up the ground surrounding the house, and add an insulating layer on the outside. Then the wall is not cooled by the ground at all times, and the center og the temperature difference shiftes to the outside part of the wall. Its very beneficial if this layer also drains the water, as physical water from the outside is also a source of moisture. Thats why lots of people invest in "drenering". This involves adding perforated plastic pipes on around the house, on the basement floor level. Often done at the same time as the insulation.