Mold is usually a direct result of moisture. Mold spores are everywhere, but when they find a nice, moist spot to settle, they can start to grow and spread, leading to a serious situation if this happens inside of your home.
There are many sources of moisture, ranging from leaks to wet clothes, but one of the more likely places for you to have mold issues is in your bathroom. Your bathroom can have water sitting in an inopportune place for long periods of time, creating ideal locations for mold to take root.
Once mold has started growing in your bathroom, it can spread quickly unless something is done about it.
So what should you do to prevent it? Ventilation is very important. You should air out your bathroom regularly if it is at all possible. You should also dry up spills, and avoid having wet towels piled up on the floor.
Your ceiling fan also needs to function properly to vent the moisture out of your bathroom.