Space stations orbit the Earth every 90 minutes.   The spend around 45 of these in the sunshine getting hot, and then spend the same amount of time in the Earth's shadow, getting cold.   This cycling can cause water in the damp air from respiring astronauts, to condense onto the cold surfaces much as it does on Earth.   Although space stations are insulated to cope with this cycling, condensation still occurs.  If this is in an inaccessible position surrounded by stagnant air, fungal spores and bacteria can propagate very quickly.   Within perhaps a month, they will be visible as green-grey blotches a few millimeters across.  If unchecked they will spread to form a thick mat of filaments.   This will release increasing numbers of spores into the air system and these will contaminate other areas of the Space Station.   They could affect crew respiration, infect  food, contaminate biological experiments and damage the very structure and systems of the Station.

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