Mould
Mould needs four things to grow - oxygen (in the air), food (like wallpaper or paint), suitable temperatures (heating levels) and moisture (from condensation, rising/penetrating damp or plumbing leaks).
Dealing with mould growth
Cleaning mould can be hazardous so wear protective gear (mask, gloves and goggles) to avoid contact with spores and cleaning products. For mould on walls use white vinegar, baking soda, or a mix of both.
Remove excess mould with a damp cloth and discard it. A tea tree oil solution (3-4 drops in 2 litres of water) can also be used; test a small area first.
Don't brush mould as this releases spores.
For washable/non-porous surfaces, use a commercial fungicide (with protective gear). After clearning, redecorate with fungicidal paint, not regular paint.
Untreated mould can damage your property and belongings.