Water leaks and dampness can cause toxic mold leading to respiratory symptoms, allergies, asthma, as well as immunological problems. These can be spread throughout an environment via the ventilation system.  The following articles can help you learn about these dangers.

http://www.nolo.com/legal-encyclopedia/toxic-mold-basics-29685.html

http://www.grandparents.com/health-and-wellbeing/health/indoor-air-pollutants

The dangers of sewage leaks and the need for professional testing and treatment.

http://inspectapedia.com/septic/Sewage_Contamination.php