Moulds (e. g. Alternaria alternata) reproduce by producing large numbers of microscopically small spores, which are adapted for wind dispersal and may remain airborne for long periods. Spores can begin growing on suitable materials like food, wall paper or wood under favourable conditions, forming large colonies or mycelia. Moulds require moisture for growth and can be found on moist walls, especially bathroom walls.

Useful tips on what to do

Frequent airing of rooms to reduce moisture levels by patch airing
(3-4 times per day, 5-10 minutes)
Remove affected materials and eleminate the source of moisture
Remove house plants