Disease

Diseases are negative conditions that are contracted from enemy attacks, curios, traps, and occasionally as a random quirk at the end of a dungeon. You can only get a disease at the end of a dungeon if your hero's resolve level is at least 2. If so, the chance of getting a disease is 32% - 0.33 * disease resist but at least 5%.

Effects
Although most of the diseases are harmful, Rabies is quite beneficial because it grants +15% DMG while only reducing -10 ACC. You can get it while fighting Rabid Gnashers. The Fits can also be useful on the Occultist and Vestal, as the +3 SPD it adds can be more useful than the lost 5 ACC and CRT, especially given that healing skills are not affected by Crit chance.

Diseases last until removed. They can be cured in the Sanitarium for some gold (where there is a chance of multiple diseases being cured at once depending on upgrade level) or by using the right camping skills from the Grave Robber or the Plague Doctor (which will cure all the diseases on the targeted hero). Additionally, the Flagellant can cure one of his own diseases, not the other characters, with one of his camping skills.

A hero can have at most 3 diseases at once; if they acquire a new one, it replaces an old disease picked at random.

The Crimson Curse
The Crimson Court DLC adds a new disease known as the Crimson Curse. Usually it can only be contracted by fighting Bloodsucker enemies, and it has several unique properties, such as conferring debuffs that get worse over time. Any character with the curse cannot have any other diseases, and any disease they had will be removed upon acquiring the curse.

Disease resistance
Heroes' resistance to diseases is one of two resistances that isn't increased by Resolve Level, the other being Death Blow resistance.

The list of heroes by their disease resistance, from highest to lowest:

Trivia
The Carrion Eater and Crone have the only effects that increase Disease chance at higher levels.