Player Core Chapter 8: Playing The Game Afflictions

Format

Whether appearing in a spell, as an item, or within a creature's stat block, afflictions appear in the following format.

To see how a disease works, let's look at severe radiation sickness.

Severe Radiation Sickness (disease) Exposure to severe radiation causes fatigue and nausea. You can't reduce your sickened condition while affected by severe radiation sickness; Saving Throw DC 22 Fortitude; Onset 4 hours; Stage 1 2d6 poison damage and sickened 1 (1 hour); Stage 2 2d6 poison damage and sickened 2 (1 hour); Stage 3 2d8 poison damage and sickened 3 (1 hour)

If something like a leaky reactor exposed you to severe radiation sickness, you would attempt an initial Fortitude save against the listed DC of 22. If you fail, you advance to stage 1. Because of the onset time, nothing happens for 4 hours, but once this time passes, you take 2d6 poison damage and become sickened 1. As noted, you're unable to reduce the sickened condition. The interval of stage 1 is 1 hour (as shown in parentheses), so you attempt a new save after 1 hour passes. If you succeed, you reduce the stage by 1, recovering from the disease. If you fail again, the stage increases by 1 to stage 2, and you take 2d6 poison damage and become sickened 2.
If your initial save against the radiation sickness was a critical failure, after the onset time, you would advance directly to stage 2. After the 4-hour onset time, you would take 2d6 poison damage and become sickened 2. Succeeding at your second save would reduce the stage by 1 to stage 1, and you'd take another 2d6 poison damage. Failing the second save would increase by 1 again to stage 3.
If you reach stage 3 of the disease, either by failing while at stage 2 or critically failing while at stage 1, you'd take 2d8 poison damage and be sickened 3. If you failed or critically failed your saving throw while at stage 3, you would repeat the effects of stage 3.
Since the disease has no maximum duration, you probably should seek magical or medical aid to help you recover—and find a way to fix that reactor!

Name and Traits

Source Player Core pg. 422

The affliction's name is given first, followed by its traits in parentheses—including the trait for the type of affliction (curse, disease, poison, and so forth). If the affliction needs to have a level specified, it follows the parentheses, followed by any unusual details, such as restrictions on removing the conditions imposed by an affliction. If no level is listed, the affliction matches the level of the creature, hazard, or item causing the affliction.

Saving Throw

Source Player Core pg. 422

When you're first exposed to the affliction, you must attempt a saving throw against it. This first attempt to stave off the affliction is called the initial save. An affliction usually requires a Fortitude save, but the exact save and its DC are listed after the name and type of affliction. Spells that can cause an affliction typically use the caster's spell DC.
On a successful initial saving throw, you're unaffected by that exposure to the affliction. You don't need to attempt further saving throws against it unless you're exposed to the affliction again.
If you fail the initial saving throw, you advance to stage 1 of the affliction and are subjected to the listed effect. On a critical failure, after its onset period (if applicable), you advance to stage 2 of the affliction and are subjected to that effect instead.

Onset

Source Player Core pg. 422

Some afflictions have onset times. For these afflictions, once you fail your initial save, you don't gain the effects for the first stage of the affliction until the onset time has elapsed. If this entry is absent, you gain the effects for the first stage (or the second stage on a critical failure) immediately upon failing the initial saving throw.

Maximum Duration

Source Player Core pg. 422

If an affliction lasts only a limited amount of time, it lists a maximum duration. Once this duration passes, the affliction ends. Otherwise, it lasts until you succeed at enough saves to recover, as described in Stages below.

Stages

Source Player Core pg. 422

An affliction typically has multiple stages, each of which lists an effect followed by an interval in parentheses. When you reach a given stage of an affliction, you're subjected to the effects listed for that stage.
At the end of a stage's listed interval, you must attempt a new saving throw. On a success, you reduce the stage by 1; on a critical success, you reduce the stage by 2. You're then subjected to the effects of the new stage. If the affliction's stage is ever reduced below stage 1, the affliction ends and you don't need to attempt further saves unless you're exposed to the affliction again.
On a failure, the stage increases by 1; on a critical failure, the stage increases by 2. You're then subjected to the effects listed for the new stage. If a failure or critical failure would increase the stage beyond the highest listed stage, the affliction instead repeats the effects of the highest stage.