*Trigger Warning* This blog post talks openly about suicidal thoughts and other matters that could be triggering so please don't read any further if you are likely to be affected.
Everybody gets intrusive thoughts. It's the content and intensity of them that can turn them into a symptom of mental illness. Intrusive thoughts are, like the name suggests, a thought that intrudes into a person's mind that wasn't initiated by them. For example, a person could be standing at the top of a flight of stairs about to descend, when the thought, "I'm going to trip and fall down the stairs!" pops into their head. This is a fairly typical intrusive thought and provided the thought doesn't persist to a point where the person is too afraid to walk down the stairs, this isn't a symptom of mental illness. It's just a fleeting thought that the person can dismiss easily.
