Disease

[General Tacticus]

I don't know about the Harvester of Sorrows, but the Demon Lord Pazzuzu also spread disease, and in his case it kills the population of the province he's in. I don't know if it also affects troops, I didn't keep him long enough near mine to find out

As for deadlier, I vote Burden of Time because it is world-wide, although the affliction gained is random.
Starvation can also be a real killer, since a unit that starve two turns in a row get diseased (100% probability). It works as PvK said, units with special survival (forest, swamp, mountain, etc.) are always the Last to starve in an army.

[E. Albright]

Originally posted by PvK:
Note that a regeneration item can forestall death from disease, but the victim will eventually become an afflicted wreck.

Are you sure of this? In Dom1, I recall a time when I had one of the Eternal Knights get diseased, and a good thirty turns later, it had no afflictions beyond that...

[Taqwus]

Check whether the Eternal Knights have a 'recuperation' flag. Recuperation heals diseases like any other affliction, although it may not be fast enough to save the unit in all cases.

[Saber Cherry]

Disease is disease. Bane blades / banefire / "fast-acting diseases" are different, and either kill during combat or do not kill. But the disease affliction does not vary in magnitude.

[Nagot Gick Fel]

Originally posted by PvK:
If a unit gets a "starving" hit who is already starving, he gets diseased.

Starving units may get diseased even if they weren't starving the previous turn.

[E. Albright]

Originally posted by Taqwus:
Check whether the Eternal Knights have a 'recuperation' flag. Recuperation heals diseases like any other affliction, although it may not be fast enough to save the unit in all cases.

Couldn't have been that. I wasn't clear enough. After a lengthy delay, the diseased, regenerating Knight was still infected, but the disease was its sole affliction.

The creatures: Harvester of Sorrow, Pazazu, and Mabakeil can all cause a virulent disease that kills population but also diseases commander/troops in that particular province (though the chance is low). This includes friendly units as well.

The Bane Venom charm works the same way except it doesn't cause disease in commander/troops (except the one it's equipped on)