Attics lack the proper ventilation to keep moisture out of them. When your attic becomes infected with moisture it has the ability to produce mold growth rapidly. Since there is no where for the moisture to be ventilated, it often soaks in the walls and leaves them damp and causes rot to your home. In the summer when your attic becomes really humid and hot from lack of airflow, mildew is prone to grow.
If you think that moisture is trapped in your attic it is crucial you hire a professional to properly ventilate it.