A lot of people make the mistake of blocking up air vents or any place there is an air draught in a property. It could be an air vent, a trickle vent on a double glazed window or even draugh proofing on windows in winter. This  and/or drying washing on radiators or not having ventilation in a bathroom is the main cause of condensation which causes fungus - mould growth in properties. 

This is often very common in rented property. It is important that whoever is living in the property is away of this issue, as it does not take long for fungus to grow and spread.

If you already have this problem you will need to address the cause and look at how the space is being used and what ventilation is present and look to install some air vents if there are none (there are simply vent or electrical vents that read the temperature in the room and adjust accordingly to ensure comfort levels are maintained). To remove the existing mould you will need to wash done the walls with a specialist anti-fugus spray or wash that you can purchase from any major building store like B& Q or Homebase.