Moisture is mold's best friend and it thrives between 40 and 100 degrees Fahrenheit which is why it is commonly found in homes.  Mold spores float in the air and can grow on virtually any substance with moisture including tile, wood, drywall, paper, carpet, and food.

Moisture control and eliminating water problems are key to preventing mold. Common sources of moisture can be roof leaks, indoor plumbing leaks, outdoor drainage problems, damp basements or crawl spaces, steam from bathrooms or kitchen, condensation on cool surfaces, humidifiers, wet clothes drying inside, or improper ventilation of heating and cooking appliances.

Control the moisture problem
Scrub mold off hard surfaces using soap and water or other cleanser; dry completely
Do not paint or caulk moldy surfaces
Discard porous materials with extensive mold growth
Avoid exposing yourself or others to mold
Periodically, inspect the area for signs of moisture and new mold growth
The EPA suggests that if the moldy area is less than ten square feet, you can probably handle the cleanup yourself.  If the affected area is larger than that, find a contractor or professional service provider. 

• Increasing ventilation in a bathroom by running a fan for at
• least 30 minutes or opening a window can help remove moisture and control mold growth.  
• After showering, squeegee the walls and doors. Wipe wet areas with dry towels. 
•  Cleaning more frequently will also prevent mold from recurring or keep it to a minimum.

A simple solution to clean most mold is a 1:8 bleach/water mixture.  Since homes have thermostatically controlled temperatures and water is used all day long in the kitchen and bathrooms, the environment is conducive to mold. 

See Ten things you should know about mold written by the EPA.