I wrote earlier how it is not uncommon for houses to have mold problems here. At a break during a meeting, I was talking to a colleague who had some horrible mold problems in her brand new house that she had rented (again, I don't understand why Germans don't come up with updated building techniques that help mitigate mold). She had a horrible episode with mold: she took a trip back to the States for three weeks and wasn't home to air out the home daily as recommended. She had informed her landlord of the trip and wanted to take tenancy of the house after the trip, but he really wanted her in there (and her rent money) before she left and said the house should be fine.
Imagine her horror when she came home from the trip and found the floors and wood trim buckled from humidity and covered in mold! It was absolutely horrible. If I remember correctly, this didn't even happen during the summer; I believe that she said it was in November.
Another person has also had continued mold problems that even required extensive renovations of the bathroom. She hasn't been feeling well on and off and has developed allergies; she believes it could be because of the mold. I cautioned her about it because I personally know one person who almost died from exposure to black mold. It's a very serious matter, for sure, and it's very hard to get rid of it.