Immunity to sickness (also known as sickness resistance) is a property that protects your character against both disease and food poisoning. When you have the immunity, you never need to use a unicorn horn or spell to cure sickness. In vanilla NetHack, this property is very rare: you must polymorph into a fungus or a ghoul to have immunity to sickness. The property is more common in SLASH'EM, where two roles, the Necromancer and the Undead Slayer, start the game with innate immunity to sickness. Disease attacks of monsters like Juiblex do not affect characters with immunity to sickness.