As a Realtor and extremely mold sensitive individual, I have encountered many homes over the years with mold and mildew issues. Mold is subversive and subtle and not uncommon in our Southern Appalachian climate. A home that is not maintained properly can quickly develop mold issues. Some are easy to fix (NEVER scrub with bleach or remove yourself -- the spores will spread when disturbed) and other problems cost thousands of dollars to remediate.
I have long said that "bad things happen to vacant homes." I think that many people assume that I am talking about vandalism. But I can't actually remember ever showing a home that had been vandalized. What I mean is that when left to her own uncontrolled devices, Nature quickly takes over. The primary method she uses is mold.
And mold is no small problem.
Again, it must be remediated by a professional to prevent spreading the spores throughout the home and the HVAC system. This can be costly.
Worse are the side effects of mold. Some people, like me, are so sensitive to mold that we absolutely cannot be in a moldy home for even a few minutes without suffering varying reactions to it. And here is an important point: ALL MOLD IS TOXIC. Black mold gets all the press, but other molds like Aspergillus and Penisillium are also health-threatening molds. To sensitive people, all molds are a threat to health and well-being.
All that being said, I have noticed that many, many bank-owned foreclosed homes, which have often been vacant for months if not years with the power off and no ventilation, have mold problems. The home that seems like a "deal" may actually cost you thousands to remediate or your health if you don't remediate properly.
I heard a piece on NPR this morning that says it better than I can. You can listen to or read the story here: