From Diablo Wiki
Disease was to be a type of damage in Diablo 3. It was similar to poison damage, except the effect would have been to cause a damage debuff, causing diseased targets (players or monsters) to take extra damage while dealing reduced damage.
This effect was removed along with all of the other secondary damage effects (save chilling/freezing from cold damage) during late development, pre-release.
Reaper of Souls is set to reintroduce many damage types and secondary effects, and disease may be one of them.
Diablo III Damage Types
any change in the range of damage types to include holy, shadow, voodoo or anything else to match the classes? —Scyberdragon
Damage types will likely change some but are currently Physical, Fire, Lightning, Cold, Poison, Disease, Arcane, and Holy. —DiabloFairly significant in their difference, but both countered with a single resistance. Damage/resists aren’t design complete though.—Diablo
Whats the difference between poison and disease damage?—WickedBubba
Disease has a damage debuff (both intake and output), and poison has a health debuff (regen/heal). Subject to change of course.—Diablo
Is there much difference between poison and disease? They’re both internal maladies that hurt over a period of time.—Grug