Mold generally forms in moist, warm locations. Condensation in your windows can create these optimal conditions that dangerous mold needs to thrive. If you have not cleaned your windowsills in a while, you may notice dark brown or black spots. Yes, that would be mold. It is particularly important to get this cleaned up because it can cause dangerous respiratory infections.
Of course, these are just a few of the many potential health benefits that come with dirty windows. So, make sure that you get yours looked at to ensure a safe and healthy home or office space.