Air conditioning system condensation pans under indoor units provide a perfect environment for mold and bacteria.  If accessible, be sure to check all pans regularly and thoroughly bleach-clean them as needed.

Ground water that is not directed away from your home may penetrate the foundation and/or the crawl space walls and will provide mold and mildew with the moisture they require to thrive.  Pipe downspouts away from the house and maintain a 15 percent positive grade away from your foundation.

Exterior decaying wood may lead to water damage inside the home, creating an inviting environment for the growth of mold.  Be sure to check all wooden surfaces inside and outside of the home that are subject to frequent or constant moisture exposure.  Lower reaches of wood siding in contact with soil, improperly designed window and door ledges and porches are especially vulnerable to decay.

Be sure to inspect the inside of the home for water damage corresponding to any found on the outside of the home and make sure to fix the source of the water leak.

Faulty or missing flashing allows rainwater to enter your home and settle in seldom visited areas such as attics and insulated cavities.  Check and maintain flashings annually.

Unfinished, dirt-floor crawl spaces allow ground moisture to rise and permeate house framing and insulation.  Such crawl space surfaces should be sealed by placing polyethylene sheeting over them and taping all seams and edges.  Keep crawl spaces well ventilated.

Install proper attic ventilation, e.g. soffit vents, to increase airflow and decrease humidity in the attic (this will also extend the life of your roof).

Bathrooms and laundry areas that are unventilated or poorly ventilated encourage the growth of mold and mildew.  Venting (including dryer vents) should always be to the outside and not to an interior area such as an attic or garage.  Install bathroom exhaust fans in rooms with or without exterior windows to decrease humidity.

Humidifiers and improperly vented fuel-burning appliances can raise the relative humidity in a room.  Shut off offending appliances at first signs of condensation on windows or other surfaces.  Check venting systems to ensure all potentially harmful flue gas exhausts to the exterior.

Dehumidifiers and air conditioning should be used, especially in hot, humid areas, to draw moisture out of the air.  To be sure these appliances don't themselves become sources of biological pollutants, clean or change the air conditioner filter to prevent dirt and mold buildup.

Major systems, such as furnaces, heat pumps and central air conditioners, should be inspected and cleaned annually before seasonal use.

Be aware of the hazards of synthetic stucco siding: In the last 10 years, more and more builders have used a synthetic stucco or Dryvit product known as EIFS (Exterior Insulation and Finish System), which has been well received by consumers.  The product was seen as cost effective and attractive, but has now proven to be extremely problematic and expensive to fix.  If the material is not applied properly, seepage can get trapped behind the siding causing severe  deterioration.  This condition not only deteriorates the framing of the home, but creates an environment that is extremely conducive to the growth of toxic molds.  Replacement of the improperly installed stucco can cost in the tens of thousands of dollars.  If you fall in love with a house with EIFS, just be sure to have a home inspector trained in EIFS inspection take a detailed look at the home.  Be prepared to walk away in the event the inspector sees problems.

Source:housemaster.com