The risk of getting a negative effect from a disease is reduced by this value. 100 means the unit will be totally unaffected, just like an undead being.

The value is a percentage, and it is rolled fresh every time a disease would do something. The game rolls a number from 0 to 99, and below the value the disease event is cancelled outright. Almost every creature that has the ability has it at 100, so it usually reads as plain immunity. The bless picks Half Dead and Strong Blood are the exception: both grant 80, which cancels four events in five and cures nothing.

• Catching a disease. The Plague aura and the spell Plague, the disease a Reaper spreads through the province it stands in, attacks that infect their target, and the random events and dominion effects that disease the units of a province. For Plague this is a second saving throw on top of the Magic Resistance check and is rolled after it, so either roll alone is enough to escape.
• Suffering a disease already caught. The roll comes before the monthly damage, and a success skips that month entirely.
• The battlefield plague. The damage and fatigue the Plague effect inflicts round after round is rolled against Disease Resistance as well.

The Miasma dominion of MA C'tis is an exception of its own: it reads the value but never rolls it. Anything below 100 is diseased all the same, so the 80 from a bless gives no protection there.

The one thing it does not cover is the disease clouds of a Plague Carrier. Those inflict the affliction on a failed Magic Resistance check and never look at Disease Resistance, so even a creature with 100 becomes diseased when it stands in one. What saves it is the next point.

Disease Resistance of 100 removes the disease itself. A diseased creature that has it loses the affliction at the start of the next turn, before any damage is rolled. So a Plague Carrier's cloud can leave such a creature diseased for the rest of a battle, and the affliction is gone by the following turn without ever having cost it anything. A value below 100 does not cure: a creature with 80 that catches a disease keeps it, and rolls against it month after month.

Four kinds of creature count as Disease Resistance 100 whatever their listed value:

• Demons
• Spiritform creatures
• Stone beings
• Inanimate creatures that are not undead

Undead are a separate case. They are not counted as resistant, and a disease cloud marks them diseased like anything else, but a diseased undead is skipped by both the monthly damage and the battlefield plague, so the affliction never costs them anything either.

Having no resistance has one visible consequence. Only a value of 100 clears a disease, and an undead creature reaches 100 only if it is also on the list above, in practice by being Spiritform or a stone being. No undead in the base game is also a demon. Every other undead that catches a disease keeps it until a Disease Healer removes it: the mark stays on its sheet, and it never does anything.