Mould or mildew spores are everywhere but only develop if three conditions are met. They are:
1) A food source, usually the wood components;
2) The right temperature (40-140 degrees F);
The easiest thing to control is moisture. The main source in the attic comes from moisture-laden air from the living space. It enters from uninsulated ceiling light fittings, any gaps in the vapour barrier or un-weather sealed attic hatches. Any moisture that does enter should be removed by adequate air flow in the attic space, hence the importance of the ventilation provision.