If there is moisture than there can also be mold. The coils of an air conditioner, a faulty humidifer of a central heating system, or just high humidity can give mold spores enough moisture to thrive.

Mold spores are naturally occurring and are an important part of the environment outdoors. Indoors these tiny particles can cause a great many problems. Spores can enter indoors on our clothes, on our pets, or come in through vents. They get blown around by heating or cooling systems until they find moisture, oxygen and a food source. In a small space of time the beginning of an infestation is underway.

Exposure to mold can cause an allergic reaction in individuals with or without sensitivity to other allergens. Sneezing, runny nose, eye irritation, and a skin rash are a few common symptoms that may occur during or after exposure to mold.
People with asthma may notice an increase in respiratory difficulty. Immune-compromised individuals may be more acceptable to infections from mold.

U.S. EPA’s web portal on mold

New York Times Article on Mold

The musty smell of a house or building could be an indicator of a mold problem. Mold isn’t always visible. Often mold only becomes visible after an infestation is at a critical point. Duct work, the back side of drywall, and behind furniture are just a few places mold could be hiding.
Disturbing potential sites of mold growth can lead to a release of mold spores and only increase the areas affected. If you suspect a mold problem contact a professional to test your home or business.

The size of the mold affected area will determine the cleanup effort needed to rid your home of mold. The EPA recommends mold infestations over 10 square feet be treated by a professional. Areas smaller than 10 square feet may be small enough for an individual to handle. Wear protective material such as gloves, goggles, and masks.

Adequate ventilation is key to preventing the buildup of condensation. Exhaust fans or opening windows in areas prone to excessive moisture, such as kitchen and bathrooms, assure the release of moisture laden air before it can cause problems. Here a few other tips for helping to reduce moisture indoors: