Mould needs moisture + poor ventilation.
Common sources of moisture:
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steam from showering,
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cooking without lids or extractor,
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drying clothes inside,
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breathing (especially in small, closed rooms),
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leaks from pipes or roofs (less common, but serious).
If the air is humid and the room is not ventilated, moisture sits on cold surfaces (corners, windows, outside walls) and mould starts to grow there.