What is the air like in your bathroom after you take a shower? Most likely it is steamy and hot, unless you prefer taking cold showers. All that hot humid air can do great damage to your bathroom, however. It can help mold get a foothold in your bathroom, as well as, other water related damage. This is why your bathroom should have an exhaust fan clearing that air out.
Unfortunately, not all bathroom fans work the way they are intended to. There are many shortcomings that could come from the way they were installed, the way the ductwork was routed or even from the age of the fan itself. You should make sure that your fan works well, and that it vents to the outside of your home, not into the attic or a crawl space since you would just move the problem to a different area of your home.