It should have strong symbolic or metaphorical possibilities. If there’s a metaphor connected with smallpox, I don’t want to know about it. Smallpox was hideous in both the way it presented and the disfigurement it left without really offering any constructive symbolic possibilities.
I do agree that illnesses in literature can have a symbolic or metaphorical value that can make them all the more valuable to include. That being said I don’t think you have to be particular to an illness in order to have said symbolic or metaphorical value. A good author will find a way to give something, anything meaning and importance.
