From Harmless Flies To Existential Threat in 10 seconds
Here is an example of how someone who is preoccupied with a fear of being infested by parasites can be triggered by a few fruit flies that have appeared in the kitchen overnight. Notice how, even though the cause was most likely an over-ripe piece of fruit, the person's first automatic thought (that it's an infestation) jumps straight to a worse-case conclusion, which rapidly triggers more catastrophic thinking, resulting in a vicious cycle of hypervigilance, anxiety, stress, and unhelpful OCD behaviours.
Theory A vs Theory B
If you were to apply the Theory A vs Theory B method to the same situation, it might look something like this: