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.