Since then I've been lucky enough to discover the work of Richard Dawkins. His 1991 article "Viruses of the Mind" explains the concept of an idea being analogous to a virus that infects the mind.
Like computer viruses, successful mind viruses will tend to be hard for their victims to detect. If you are the victim of one, the chances are that you won't know it, and may even vigorously deny it. Accepting that a virus might be difficult to detect in your own mind, what tell-tale signs might you look out for? I shall answer by imaging how a medical textbook might describe the typical symptoms of a sufferer (arbitrarily assumed to be male).
It's worth reading his description of those symptoms. Check out his article if you get a chance - especially if you think you might have been infected :)