High relative humidity at the surface of wall allows mold to grow. Winter mold growth is caused by building surfaces being at the dew point of the air in the home. This is like a can of cold pop sweating. Two choices, reduce the indoor humidity (dew point) or raise the wall surface temperature. Increasing the fresh air ventilation reduces the indoor dew point when it's cold outside (below 50^F).
The wall surface is warmed by keeping mirrows, couches and dressers away from exterior walls, raising the indoor temperature, or improving the insulation. If this is summer problem, reduce the indoor humidity below 50% and don't cool the home below 75^F. This usually requires a dehumidifier. Do a web search for "ventilating dehumidifier".