Winds of Chaos

I want my PC's to run into some Nurgle cultist and Nurgle Chaos Warriors at some point (rivals to the Tzeentch main villains in my campaign).
But the diseases these opponents carry, wouldn't it cripple my PC's after contact? How would you play this out?
Maybe more "between adventures" downtime and lost endeavors as they recover?
Or maybe even lost Fate points or PC deaths to disease? (some of my players might find such a death anti-climactic)

Any thoughts?

Make up a disease that doesn't kill them but debilitates them in some way. Maybe it decreases some important stats until a cure can be found ... in a city, village, or forest where you'd like to direct the players to go next.

Wonderful idea! Thanks!