You pour raw necrotic energy into a target forging disease particles in the creature.  If the target fails its saving throw, it is affected by a mental of physical disease you choose with a save DC equal to your current spell save DC or lower.  The disease progresses as normal and can be removed as normal.

If you target a creature who is already afflicted by a disease, the creature must instead immediately succeed on a Fortitude saving throw or worsen one step on the condition track of the disease.  This saving throw does not count for their successes needed to end the condition nor does it count for their daily saving throw against the condition.