Mold 

requires a nutrient source, proper temperature and moisture to grow. Mold does not require light to grow. Mold is a saprobe meaning that it lives on dead organic materials. It does not produce food, but instead adsorbs nutrients by breaking down hydrocarbons. As such it will grow on any organic building material such as paper, adhesives, resins, etc. It will even grow on the patina of dust that collects on surfaces. The dust in office buildings consists primarily of paper dust and skin cells so that it provides an adequate nutrient source for mold growth. Nutrients to support mold growth are ubiquitous in the building environment. The temperatures required for mold growth are in the same range as indoor building environments. The pervasive nature of nutrients and a temperature range suitable for mold growth leave control of moisture as the only practical way to control mold growth.

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Bacteria

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Bacteria found in indoor environments typically come from human sources (skin and respiration) or from the outdoors. Like mold, most of the bacteria found in the air in buildings are saprobes, meaning they grow on dead organic matter. As far as building envelopes are concerned the primary concern is about bacteria colonies that may grow in damp areas.