Why does mold grow?

In order for mold to grow, it needs an environment that contains organic matter for the fungus to feed on
and a sufficient source of water to maintain cellular functioning.  Additionally, each type of mold has a
minimum, optimum, and maximum temperature for which it will grow, but most grow within 60-80
degrees Fahrenheit.  Mold is essential for the sustenance of life because it breaks down dead organic material;
if it weren’t for mold, decomposition would not occur!  However, mold becomes a problem when it is found
indoors.  There are numerous organic building materials used in construction and everyday life that make
excellent sources of food for mold spores.  If airborne mold spores land on damp indoor materials like wood,
paper, fabric, carpet, or drywall, they can continue to grow and pose health problems for the occupants.  It
should be noted that even if a surface isn’t wet to the touch, it may still contain moisture content sufficient
for fungal growth—this is where a moisture inspection comes into play.  Since organic matter is prevalent in
homes and the temperatures required for fungal growth are similar to those comfortable for humans, the
easiest way to control fungal growth is by eliminating any sources of moisture.  So the idea is to get rid of the
moisture source, then get rid of the existing mold.  Water intrusion from the outdoors is the most common
moisture source for mold in the home and can occur through leaks in the roof, walls, floors, windows or
doors.  Elevated humidity can also originate from within the home, causing building materials and
furnishings to absorb the moisture.  Plumbing leaks in toilets, bathtubs, sinks, water heaters, heating/air
conditioning systems, washing machines, and dishwashers are among the most common.  Mold (or mildew) in
the bathroom is also common because poor ventilation can cause excess humidity to build up.  
http://www.epa.gov/iedmold1/moldresources.html
http://www.epi.state.nc.us/epi/oii/mold/index.html
Committee on Damp Indoor Spaces and Health.  Board on Heath Promotion &
Disease Prevention.  Damp Indoor Spaces and Health. The National Academic Press,
Richard F. Progovitz. Black Mold: Your Health and Your Home.
Phylogenetic Relationships of Memnoniella and Stachybotrys Species and Evaluation
of Morphological Features for Memnoniella Species Identification
Richard A. Haugland, Stephen J. Vesper and Stephen M. Harmon
http://www.inspectapedia.com/sickhouse/moldclasses.htm
http://healthandenergy.com/mold's_harmful_effects.htm
http://www.auuuu.org/respiratory/aspergillosis/