First eliminate all visible sources of humidity in the room. Repair or make good ventilation. Insulate the wall if the reason lies in the damp cold walls. To insulate the wall also use the Styrofoam that stick to the Wallpaper. The most vulnerable places sell special insulating wedges.
Once you have found and eliminated the cause of the mildew, take care of removing the old one. Very carefully clean off old mold with a brush. A good solution to remove mold spores can be done at home: boric acid, white vinegar, hydrogen peroxide and water. Scrape with a reserve area with the old paint, whitewashing or Wallpaper. When cleaning surfaces, use gloves and a mask, also try to limit the surface to be cleaned so mold spores don't spread to nearby areas.
Dry the wall. If it is concrete or brick, can be dried with a blowtorch. Wooden wall dry when the fan or heater.
Once the walls are dry, soak them with a special substance that kills mold spores and prevents it from multiplying. You can use the following antiseptics when dealing with mold: "Senezh-antiplesen", "Anti-In", "Teflex-antiplesen", the drug against mold "Mipor". Before using be sure to read the precautions listed on the packaging. Follow these guidelines.
Now you can re-glue Wallpaper, paint the walls or cover them with any other material. When painting walls, use high quality ink, mixing it with an antifungal drug. If the layer of fungus was thick, it is best to re-Prime the walls.