No. No, it's not working. Which begs the question: what the hell was I testing?
Seems like you can only delay a select few effects, and not delay them for too long, or the engine just discards them.
Is it possible you were testing the disease by advancing game time through ctrl+t? I heard somewhere that certain delayed effects may become bugged if you actually save and reload the game (6 days is probably more than one playing session) - I know from my own tests that monk fists can act strangely upon reload if they have any attached equipping effect.
If you want the disease to cause delayed and certain death, why not just apply 25 constitution damage through the disease effect?