The most hazardous problem associated with a roof leak is the propagation of mold growth. Mold grows quietly with no help. It only requires a slight amount of water, and still air. The problem lies therein: mold will slowly grow until it overtakes all surfaces available, then produce spores that can easily infiltrate the attic, the insulation, the ceiling and the HVAC system, where it can then be dispersed throughout the home. Mold can grow in carpet, furniture, and even clothing. Mold spores continue to reproduce until it gains sufficient number to cause serious health problems. Health problems can result from mold spores invading the respiratory system, and can result in long term allergic reactions, asthmatic symptoms, skin rashes, and other serious health concerns.
As roofers in the trade with 30 plus years, we have witnessed many people that have become very sick from mold growing in their attic. They didn't know that their plywood is rotten, or that the roof is even leaking. They only knew that they were feeling well, and that the house seemed rather humid. Usually upon inspection of the attic, it is immediately obvious that the cause of the humidity in the house is from a leaking roof. We typically remove all of the plywood and insulation in the attic, spray down the trusses with a bleach solution, reinstall new insulation and new plywood. Only then do we know that we have removed most of the mold spores.