In behaviorism, negative reinforcement involves the removal of an unpleasant stimulus when a desired behavior occurs. It differs from positive reinforcement as the stimulus is taken away rather than given when the behavior happens. The principles of negative reinforcement may actually worsen a phobia.
Michael has a phobia of snakes, but works in a pet store where they are sold. Over time, he's become such an expert in bird care that he is promoted and now only works in the bird department of the store. This is a negative reinforcer of Michael's snake phobia; as he does not need to interact with snakes, he does not need to work to address his snake phobia, allowing it to only be perpetuated.