Water seepage and high indoor humidity are the sources of mold and mildew in the home, both of which can lead to a variety of health problems. Older, poorly insulated buildings are a magnet for them. Mildew is visible, but mold isn't.
Mold reproduces by means of tiny spores that float through the air. Exposure to mold can irritate the eyes, skin, nose, throat, and lungs; inhaling or touching mold or mold spores can cause immediate or delayed allergic reactions like sneezing, runny nose, red eyes, skin rash, or asthma attacks in asthmatics who are allergic to it.
Additional Resources
Mold Guildelines and Control (PDF, San Francisco Department of Public Health)