I hear what you are saying. When we strongly believe something with no physical evidence that is a delusion. I have gone through that for many many years but it is not as bad now. I used to be convinced I had AIDS and would die, even before I had my first sexual experience. I have been talking about these feelings with my therapist and he said they are delusions. When I finally had my first AIDS test my fears turned to fears of a brain tumor, cancer ect.... I went through a period where I thought that the world was coming to an end and once I swear I saw the sun falling from the sky!I would get myself into a cold sweat and just be sooo afraid. I never talked to anyone about these fears and so they continued to feed off each other and grow.
Since I started getting help, talking and taking medication these fears do not come as often. I think when you say them out loud to someone it actually lets some of the fear ease and the more you do it the more you realize that the fear is not the reality. Also in my case I think medications helped a lot.
It seems as though these "delusions" happen more often when I am under stress from other events in my life. I wish there was a board like this 10 years ago. I never realized that others felt the same way and felt very alone. Good luck and I hope you feel better talking about it.