Serious mental illnesses can be agony: They are as painful as physical illnesses but further bedeviling because there is no broken bone, no dead heart or lung tissue, no cancer or non-functioning organ to point to.
I don't know if it's possible to be sick with grief, but maybe that's what this impalpable scourge is. Take one heaping dose of unparalleled sadness, blend it with a few tablespoons of ennui, and you'll feel like dreck. Trust me. I can promise you that.