When you pray and get deadly sickness cured by your god, they also grant you a little temporary sickness resistance as a grace period against immediately needing it again.

An unaligned artifact amulet versus poison which also confers sickness resistance, half physical damage, and raises Constitution to 25 when worn. Tentatively named “The Eye of [something]”.

Wielding a copper object or wearing copper armor protects against an otherwise successful disease attack some of the time, because copper is antibacterial.

Sickness resistance negates the effects of potions of sickness. However, a cursed potion of sickness will now give you fatal illness instead of the normal poison effects.