i just recently discovered that there are a couple of places in our apartment where mold has started to grow. i have always thought that mold is a fairly bad thing to have growing on the walls and such, is it really that bad?
the places are under one window - a thin board supporting the window has it; behind our clothing cabinet (that one's my fault, our bed was in that place with a plastic storage bag and i spilled water there, it's kinda of a big spot; and the newest is in the corner on the ceiling.

is this something to be concerned about? what can be done about it?

Many people are very alergic to it and many ahave died...If I am not mistaken, Legionaires Diease is mold based...

Mild soap with bleach added will wash it away....Dead air, no air movement will also help it grow...
Where do you live d? Frankofurt or whatever you have listed says nothing... Warm , wet area or what....Do you use AC?

frankfurt, germany.
i sometimes use a humidifier, but only at night cause the windows were so badly fogged and i usually leave the window open. i used the thing mainly for the noise, and secondly for the humidity.
the weather here is also rather wet mostly.

What are the walls made of? Moisture is generally bad for interiors, it creates a lot of problems, not just mildew, rust too.