Many paranormal hobbyists are aware of the problem of particles in photographs. Many are aware of it but also choose to deny that it is a naturally occurring problem, and instead consider it evidence of paranormal activity.
The majority of debates about this issue revolve around the fact that dust particles, moisture, insects, etc. can be caught within a certain range in the camera's flash. Many people say that they have caught strange anomalies in daylight where they were using no flash and cleaned their lenses. But how many of you clean your sensors?
Not only can dust particles show up in the air outside of your camera, but the boogers can also get inside your camera and create dust spots on all of your photos. They also are not always detectable, as they show up better on some backgrounds than others and become more pronounced with different camera settings. Sitting just around the sensor, these tiny particles cast shadows that show up in photos and, depending on how your camera is set, can look like anything from a blur to what many refer to as "orbs".
"But...but there is a face in it!" you say. That, my friends, is called your brain on ghosts. That really is not a technical term, but it describes it quite nicely. Our minds have the ability to translate the signals sent by our vision in many different ways. This is called perception. If you spent three hours taking photographs in the hopes of catching a spirit, those patterns that your brain can't recognize is going to become something that your mind can not only comprehend, but is expecting.
Are spirits and ghosts real? I am a 100% believer, but your camera is not going to catch them--especially if you don't clean it inside and out.
For more information, examples, and step-by-step instructions on how to clean your camera sensor, visit Cambridge in Colour Tutorials.