I'm sure everyone can relate to this feeling; at least, I'm pretty sure most females can! You know when:
a) you have a headache --> you must have a brain tumour b) you have slightly swollen glands --> you must have meningitis c) your period is slightly late --> you must be pregnant (regardless of lack of sexual activity) d) you're a little out of breath --> you're about to have a heart attack e) your foot hurts --> you've got gangrene f) etc. etc.
The weirdest thing about this phenomenon is that the more convinced you become of your own terminal illness, despite a massive lack of evidence, the more determined you are not to see a doctor because that would make it real.