Fallout 4 diseases

Diseases are an integral part of Survival mode in Fallout 4.

Overview
There is a chance of contracting a disease after eating uncooked meat, drinking unpurified water, taking damage from disease-ridden creatures, and using harmful chems; diseases convey a general drawback after contraction. Items that cleanse the body of radiation also come with the side effect of a suppressed immune system, leaving one more vulnerable to contracting a disease. Only one disease (mole rat disease) can be obtained outside of Survival difficulty; otherwise, the other six diseases require Survival difficulty being active to contract.

Diseases can be cured by visiting a doctor and paying them to remove it. Alternatively, antibiotics can be used to get rid of diseases. Furthermore, prophylactic herbal remedies can be crafted at chemistry stations to lessen the chances of contracting certain diseases in the first place.

Checks
An immediate check for contracting a disease is made 30 real-time seconds after high-risk exposure. A check is also made when going to sleep. Apart from these two situations, risk exposure simply increases the probability (disease risk pool) of getting a disease at the next check. Checks are not "rolled for" unless the chance is 25%+.

All checks resulting in a new disease, clear the risk pool down to its minimum value (which depends on current Fatigue level). All checks that don't produce a new disease cause a small drain down of the risk pool. All checks upon sleeping also clear the risk pool down to the minimum, after the check.

In addition to poor sleep quality, causes of disease risk include: using chems, eating anything (but especially bad or uncooked meat), cannibalism, drinking or swimming in irradiated water, being in rain, drinking Nuka-Cola, and taking damage from feral ghouls, bugs or other infected creatures. Each event puts the player character at an increased risk of contracting an illness. High-risk events trigger a near-immediate disease check. Even if a single event doesn't trigger illness to start, each event will increase the probability of contracting an illness with each event, up until the character sleeps (when the final check is made and the disease risk pool is cleared) or gets a new disease (which also clears down the risk pool).

Contracting

 * The chance for contracting a disease is determined by a roll, depending on the current Disease Risk Pool (DRP). The DRP is a cumulative indicator of the current risk of the character becoming diseased and starts at 0, accumulating percentage points from various environmental and food/drink sources. The DRP decays at a natural rate of -1% per game hour.
 * Sources include:
 * Food always adds +1% to the DRP (but does not force a roll). Foods can also have a low disease risk (+3% to the DRP), standard (+7%), high (+12%), or very high (+20%).
 * Drinking Nuka-Cola adds +2% to the DRP (does not force a roll).
 * Chems add +7% to the DRP. These include Buffout, Calmex, Daddy-O, Day Tripper, Fury, Jet, Med-X, Mentats, Overdrive, Psycho, and X-Cell.
 * Fighting infected enemies will add +5% to the DRP if hit in combat.
 * Swimming in contaminated water will add +3% to the DRP. The effect stacks every 2.4 game hours (10% of the game day).
 * Exposure to rain adds +3% to the DRP every 12 hours.
 * When the DRP reaches the threshold of 25%, it starts rolling for diseases.
 * All items that cause immunodeficiency to add 20% to the global chance to contract the disease for the duration of the effect and add a separate +5% to the DRP.
 * If the game rolls a disease, it is added as an effect and the player character receives a grace period: They cannot contract another disease for the next 24 hours.

List

 * When the game rolls for disease, the disease die roll needs to be lower or equal to the current DRP. All diseases have a 5-90% chance of occurring (i.e., the game will randomly select among them to apply it).
 * If the player character has taken the appropriate herbal remedy, the value must also be higher than the BoostedImmunityThreshold to get this disease. This number acts as both a minimum Risk Pool threshold to even get this disease and once the Risk Pool is high enough, add some randomness to whether the player character gets this disease with each risky action.
 * Every disease has a chance floor and ceiling to be contracted, which will always drain to the floor value.

Herbal remedies
Various herbal remedies become available for crafting, purchase and as a loot in Survival mode:
 * Herbal stimulant
 * Herbal antimicrobial
 * Herbal anodyne

Each type of remedy will act (one time only per dose) to reduce the risks of any of the two specific diseases that it resists against, but only if the herbal remedy is taken before the disease check. This means taking the herbal remedy prior to sleeping, or within 30 seconds (real-time) of a high-risk event.