The height of it happened when I was nine years old. After a nightmare, I became convinced that there was a man living in our garage (which was being used as a storage room). Whenever I'd get home from school, I immediately locked myself in my mom's bedroom until she came home from work, crying and praying to God hoping that this hypothetical man wouldn't come and try to open the door. My parents didn't know about this until I climbed out the window and ran to a neighbor's house, because I thought I was about to die. But, alas! They did nothing about it, and thus, my suffering continued. What finally caused my ailment to subside, I don't remember. Over time, my fear just slowly withered away, I guess. It's not completely gone, though. Whenever I'm home alone, I prefer to stay in my own room rather than than the living room, or else I feel a mild sense of uneasiness.
And, if one day, it turns out that there really was someone secretly living in our house, I wouldn't be totally surprised...