Fear is a common feeling like happiness or sadness. It occurs when we come in contact of any unusual dangerous activities, such as ghost, murderer, any accident(natural or man made), big diseases or big financial loss, atc. atc..
We feel very weak and unsafe at that moment & our natural work of brain stops for a little little bit, so, we become confuse and can't imagine what to do. Sometimes an emergency hormone comes out from human body.
The effects of fear is different with its quantity, someone can feel normal within seconds after a small fear and someone can feel ill for days, even fear can be a reason of death too.