PTSD (post traumatic stress disorder) is an extreme form of classical conditioning. Let’s go through an example of how conditioned fear arises in PTSD. So let’s say you are driving your car and you get into a head-on collision with another vehicle travelling towards you in the other lane. Let’s say the collision was severe, causing the airbags to deploy and knocking you out unconscious. So you get treated at the hospital and barely survive. Now everytime you pass the scene of the accident, you get flashbacks about the accident as if it were happening to you again.
So this is an example of how a neutral stimulus (a part of the road) is paired with an anxiety-provoking stimulus (car accident). But with severe, life-threatening trauma, it only takes one exposure to pair it with a neutral stimulus, thereby making it a conditioned fear- a traumatic fear.
The following video explains how PTSD develops: