I sort of agree with you, but a lot of illnesses are actually just vulnerabilities to certain other things. For instance, celiac is considered a disaease, but if a celiac patient never lets a grain of wheat pass their lips, they'll suffer no symptoms. As far as I know, AIDS won't kill you if you manage to avoid ever being exposed to any other infectious agent. A clinically significant phobia of spiders would not cause you any discomfort in a spider-free environment. Why couldn't we characterize "susceptibility to assorted causes of death" as an illness of its own?