Ailment

Ailments are a type of debuff associated and applied by one or more damage types. Ailments normally distinguish themselves from other debuffs by being applied by hits, which allows them to check conditions on the enemy, and vary their effect or duration based on damage instead of having a predetermined value from a skill or modifier.

Freeze, ignite, and shock are the default ailments of cold, fire, and lightning damage respectively. Critical strikes do not inherently always apply ailments, but they do indirectly affect the chance to apply ailments due to critical damage bonus affecting the damage dealt.

List of ailments

Ignite, chill, freeze, shock and electrocute are referred to as elemental ailments because they are associated with elemental damage types. Bleeding and poison are non-elemental ailments.

Ailment	Associated damage type	Effect
Bleeding	Physical	Bleeding can only be inflicted by physical damage dealt to Life. Bleeding enemies take physical damage over time, at 15% of the physical damage of the hit that inflicted bleeding per second. Bleeding deals an additional 100% damage per second if the enemy is moving or the bleeding has been Aggravated. The base duration is 5 seconds.
Poison	Physical and chaos	Poison causes the affected target to take chaos damage over time, at 20% of the combined physical and chaos damage of the hit that applied it per second, with an explicit source of poison chance. The base duration is 2 seconds.
Ignite	Fire	Ignite causes the affected target to burn for 20% of the fire damage of the final hit that applied it, per second. The base duration is 4 seconds.
Chill	Cold	Cold damage always inflicts chill. Chill slows all actions of the affected target up to 50%, based on the cold damage of the hit. The base duration is 2 seconds.
Freeze	Cold	Freeze prevents the affected target to move or act for a duration. The base duration is 4 seconds.
Shock	Lightning	Shock causes the affected target to take 20% increased damage from all sources, based on the lightning damage of the hit. The base duration is 4 seconds.
Electrocute	Lightning	Electrocute causes the affected target's action speed to drop to 0, effectively immobilizing a target.

Damaging ailments

Damaging Ailments are ailments that deal damage over time. Bleeding, poison, and ignite are damaging ailments. The remaining ailments, Chill, freeze, shock and Electrocute are non-Damaging Ailments.

Mechanics

Ailments area family of common debuffs associated with specific damage types. Modifiers to skill effect duration affect the duration of debuffs applied by skills, but not the duration of ailments.

There is no limit on the number of different ailments a target can have at any given time.

Application by damaging hits

Ailments are typically applied by hits comprised of their associated damage types.

Each ailment is associated with one or more damage types. A damaging hit can inflict any ailments associated with the types of damage dealt. For example, a hit of physical and fire damage can inflict bleed, poison and ignite. (However, each ailment's damage/non-damaging effect magnitude is based on only the damage value of the damage types that inflicted it.) Bleed has an additional restriction: It can only be applied by attacks.

A damaging hit also carries a "chance to inflict" each ailment. This roll must be met after the hit is determined to have the correct damage type to inflict the ailment. By default:

Hits have 100% chance to inflict Chill.
Hits have 1% chance to inflict Ignite and Shock per 4% of enemy Ailment Threshold. Critical Strikes does not apply ailments inherently, but hits with critical strikes deal more damage, resulting in larger chance to apply these ailments.
The base chance to inflict all other ailments is 0%. This can be increased by many passive skills and equipment modifiers. Some skill and support gems include modifiers that grant additional chance to inflict certain ailments.

Enemies which take no damage (in general, from the hit damage, or from the damage types that can inflict the ailment) cannot have ailments inflicted upon them from hits. For example Chaos Inoculation will prevent application of poison via chaos damage, but will still allow being poisoned by physical damage.

Targets may also have immunity or a chance to avoid certain ailments. These are multiplicative with the player's chance to inflict ailments on the target; 100% Avoidance and Immunity will take priority over sources of Always inflict <Ailment>.

To summarize, an ailment is applied to the target if:

The hit damage taken consists of damage types that have the potential to inflict the ailment
The chance to inflict the ailment is successful
The ailment's effect and/or duration are calculated to be at least the minimum allowed for that ailment type
The target fails to avoid the ailment
The target is not immune to the ailment
Any other conditions are met

Application by other means

Non-damaging ailments and ailment threshold

Non-damaging ailments have the magnitude of their effect (or their duration, in the case of freeze) calculated based on the hit's damage divided by the enemy's ailment threshold. The exact formula is not known yet.

Each non-damaging ailment type also has a minimum and maximum effect: an ailment that would fall below the minimum effect is discarded and not applied, while an ailment that would exceed the maximum effect is capped at that value.

The magnitude (or effect) of an ailment is dependent on the damage the enemy takes from the hit, not the base damage of the hit. Sources of increased non-damaging ailment effect will reduce the threshold to apply a certain magnitude of an ailment, but cannot exceed the capped effect.

Non-damaging ailments that are applied by non-damaging means will use these default magnitudes unless otherwise stated (this can be verified by viewing the detailed modifier tooltip):

Chill - N/A, slow depends on rarity of enemy and number of players in the area beyond the first
Shock - 20% increased damage taken
Electrocute - interrupts the target's actions and prevent them performing any action for 5 seconds

Damage with ailments

Ignite, bleed and poison are damaging ailments. A damaging ailment deals damage over time based on the skill's base damage that inflicts the ailment. Only the damage types that can inflict a damaging ailment are used as a basis for calculating the damage over time for that ailment. From a previous example, a hit of physical and fire damage can potentially inflict bleed, poison and ignite. In this example, only the fire portion of the base damage is considered for ignite.

Damage with hits and damage with ailments are calculated separately and have different modifiers (TBC):

Modifiers to damage with ailments do not contribute to the damage of attacks or spells. Similarly, modifiers to attack damage and spell damage do not affect damage with ailments.
Modifiers to damage with weapons, melee damage, projectile damage and area damage do not apply to ailments under any circumstances.
Modifiers to damage with ailments, damage over time modifiers, and generic damage modifiers all apply to damaging ailments.
Modifiers to damage with specific ailments apply only to those ailments. Damage type modifiers apply to ailments that deal those types of damage.
Additionally, burning damage modifiers apply to ignite, since 'burning damage' simply means fire damage over time. Resistances apply to damage of the corresponding types dealt by ailments.

Critical strikes

Ailment defences

Skill gems

List of gems containing the phrase "ailment":

Active gems

Gem

Support gems

Gem
Intense AgonyIntense AgonySpell, Duration Level: 1Supports Spells that deal non-Ailment damage over time, causing them to have a shorter duration but deal substantially more damage over time with Debuffs inflicted against enemies on full life.Damage over time Debuffs inflicted on Full-life enemies by Supported Skills Deal 50% more damage Supported Skills have 25% less Skill Effect DurationPlace into a Skill's Support Gem socket in the Skills Panel to apply its effects to that Skill. You cannot have multiple copies of the same Support Gem socketed at the same time.
HinderHinderSpell, Chaos Level: 1Supports Spells that deal non-Ailment Chaos damage over time, causing damage over time they inflict to also Hinder enemies.Chaos Damage over time inflicted by Supported Skills also Hinders enemiesPlace into a Skill's Support Gem socket in the Skills Panel to apply its effects to that Skill. You cannot have multiple copies of the same Support Gem socketed at the same time.
Chaotic FreezeChaotic FreezeSpell, Chaos Level: 1Supports Spells that deal non-Ailment Chaos damage over time. Enemies taking damage over time from Debuffs caused by supported skills will suffer Freeze build up from Chaos damage Hits from any source.Chaos damage Contributes to Freeze buildup for Hits against targets taking damage over time from Supported SkillsPlace into a Skill's Support Gem socket in the Skills Panel to apply its effects to that Skill. You cannot have multiple copies of the same Support Gem socketed at the same time.
Drain AilmentsDrain AilmentsSpell Level: 1Supports Spells that deal non-Ailment damage over time, causing them to consume Ailments on enemies it applies damage over time Debuffs to in order to deal substantially more damage over time.Supported Skills Consume a random Ailment on the target when inflicting damage over time Debuffs, causing the Debuff to deal 35% more damage if an Ailment was consumedPlace into a Skill's Support Gem socket in the Skills Panel to apply its effects to that Skill. You cannot have multiple copies of the same Support Gem socketed at the same time.
Elemental FocusElemental FocusLevel: 1Supports any skill that Hits enemies, causing it to deal more Elemental Damage but be unable to inflict Elemental Ailments.Supported Skills deal 25% more Elemental Damage Supported Skills cannot inflict Elemental AilmentsPlace into a Skill's Support Gem socket in the Skills Panel to apply its effects to that Skill. You cannot have multiple copies of the same Support Gem socketed at the same time.
ExecrateExecrateLevel: 1 Cooldown Time: 8.00 secSupports any damaging Skill that you use yourself, granting it significantly higher chance to inflict Ailments on hit, but causing it to gain a long cooldown. Cannot support Skills which already have a cooldown.Supported Skills have 100% more chance to inflict Ailments Supported Skills have +8 seconds to CooldownPlace into a Skill's Support Gem socket in the Skills Panel to apply its effects to that Skill. You cannot have multiple copies of the same Support Gem socketed at the same time.
Elemental DischargeElemental DischargeSpell, Lightning, Cold, Fire, Payoff Level: 1 Cooldown Time: 1.00 sec Cast Time: 1.00 sec Critical Strike Chance: 10.00%Supports any Spell that Hits enemies, causing it to consume Elemental Ailments on hit to trigger an Elemental Discharge. Cannot support the skills of Minions.Supported Skills Cannot inflict Freeze, Shock or Ignite Supported Skills Consume Freeze, Shock and Ignite on hit to trigger Elemental Discharge Consuming Ignite deals 35% to 55% of your Intelligence as Fire Damage Consuming Freeze deals 30% to 50% of your Intelligence as Cold Damage Consuming Shock deals 1% to 80% of your Intelligence as Lightning DamagePlace into a Skill's Support Gem socket in the Skills Panel to apply its effects to that Skill. You cannot have multiple copies of the same Support Gem socketed at the same time.