The cause of mould is very much a case of warm air touching cold surface, causing condensation on the wall. this in turn dampens the dust on the wall/ceiling etc where the airborne mould spores germinate.
every case is deifferent but the causes are often just down to lifestyle of the householder, as we say mould is a symptom of a cause, find the dampness and cure the cause and dry the area and ventilate and often the problem will go away.
view my website for a bit more detail on the subject as it is explained easier than i can do here on just a few lines.
ithas2bedone.com
cheers for your interest
steve