Affliction

Characters in Darkest Dungeon are not the unwavering heroes of high fantasy to which you are accustomed. While they are certainly tough and capable fighters, they are also very human and you will need to manage their Mental Health as well as their Health. As such if their Stress Bar reaches a peak, they will respond emotionally to the horrors they encounter, by becoming Afflicted, or, more rarely, Virtuous.

Note that one risky strategy is to stack trinkets that boost virtue % up and deliberately/passively let the stress of said hero pile up. That can be done, for instance, by curio interaction, since some curios without cleansing interaction have a medium/high chance to disturb your hero (e.g. the Sacrificial Stone or the Confession Booth among others) or to a greater extent, by "cleansing" specific curios that skyrocket your hero's stress (e.g. using a Torch on a Pile of Books, or a Shovel on a Bas-Relief).
 * These interactions, which under normal circumstances are to be avoided, here can provide an easy way to reach the breaking point of your hero's sanity and trigger the resolve check. In order to obviate an unsuccessful resolve check, it would be recommended for this very specific strategy a Stress-heal heavy party. Here a Crusader and Jester combo can help through their respective Inspiring Cry and Inspiring Tune skills downright bring your hero back to null stress in 4/6 turns, depending on the aforementioned skill's level. For maximum efficiency, make the hero who has to pass the resolve check possess a self stress-heal (Leper or Abomination) in order to expedite the process to a mere 3 turns (although the Abomination cannot be matched with the Crusader, so a Houndmaster would be advisable as a replacement).

Affliction
"Madness... our old friend."

Affliction is a temporary state a Hero is most likely to enter upon reaching 100 stress. Afflicted Heroes suffer stat penalties, can act on their own, pass turns, stress out other party members, move around (disrupting your party formation), steal loot, and refuse heals and buffs. Becoming Afflicted does not immediately add stress to an affected party member or the group, however when in an Afflicted state the hero can cause serious stress damage to the entire party. Accumulating stress doesn't stop after becoming afflicted and if a Hero reaches 200 stress, they will have a heart attack, reducing their HP to 0 and bringing them on death's door. If a Hero has a heart attack on death's door, they will die instantly and permanently. Afflictions can also be cured in Town, by participating in the activities in the Abbey or Tavern, or in a dungeon, by reducing the afflicted Hero's stress to 0.

List of Afflictions

 * Paranoid
 * Selfish
 * Irrational
 * Fearful
 * Hopeless
 * Abusive
 * Masochistic
 * Rapturous

Virtue
"Many fall in the face of chaos, but not this one. Not today."

Virtue is a temporary condition of increased effectiveness that a Hero acquires after passing a Resolve Check. Virtuous Heroes have increased stats, can randomly heal themselves, reduce stress of the party or buff their teammates at the start of their turn. Becoming Virtuous reduces stress of the entire party by a small amount. Virtuous Heroes are immune to a heart attack (their stress will not increase over 100%). The virtue state will end upon returning to the Hamlet.

The base virtue chance is 25%. Characters with lower level than the dungeon they are in get a virtue chance penalty of 5% per level difference except when the difference is 6 (level 0 characters in the Darkest Dungeon) then the penalty is 33%. The virtue chance is always at least 1% and is capped at 95%.

Heroes favored with a virtue are protected from some stress-inducing events, most notably being immune to stress damage caused by an afflicted hero in the ranks, and will refuse to be anchored by the Drowned Anchorman's Heave To!, which will immediately release the hero upon reaching a virtuous state.

List of Virtues

 * Powerful
 * Courageous
 * Stalwart
 * Vigorous
 * Focused

Trivia

 * There is dialogue data in the game for a sixth Virtue, Prosperous, which would increase the value of found treasures. However, this Virtue appears to have been dummied out early in development for unknown reasons.
 * Hopeless is named "depression" in the game files.
 * The mechanic itself became a meme and it's currently the most popular meme about the "Darkest Dungeon".