Intruders are those persons who say and do whatever they can whenever they can to disrupt your happiness and wellbeing. They do so without warning. They will say and do whatever they can to cause you to feel hurt and pain. They force hurtful things on you that they know are difficult for you not to think about. Intruders will cause you to have doubts about what you can do who you can be.
When Intruders do it once, they will do it again until they think that they have you subdued, weakened, and thrown off your stride. The only signal they need is you being in their presence, for that in itself shows a vulnerability and poor judgment. You brought it on yourself by how you think. It is classic disordered thinking. It is how you think.