Disease

Most diseases are contracted as part of an event. Your colony's biome determines how common diseases are, as well as how common each disease is. However, some diseases happen situationally, regardless of biome:

Infection may occur after a bleeding injury, burn, or frostbite is inflicted. Tending the injury in a clean room reduces the chance of infection.
Lung rot occurs randomly when inhaling enough rot stink.
Blood rot and paralytic abasia are exclusively part of quests, while infant illness can only happen during birth.

Note that diseases are not contagious in RimWorld. Disease does not ever spread between colonists, though disease events often simulate "outbreaks" by inflicting multiple colonists at the same time.

The drug penoxycyline protects against Malaria, Plague, and Sleeping sickness. The disease event still happens, but colonists will not be affected.

Summary

Humans and animals can get diseases. Most diseases uses the severity mechanic. As a disease progresses, its symptoms progressively get worse. If a fatal disease's severity reaches 1.0 (100%), then death occurs. Most diseases go from minor -> major -> extreme. How fast a disease progresses depends on the disease itself.

Treatment

Most diseases can be tended to, just like an injury. Tend Quality is determined by a variety of factors, including the doctor's skill and the quality of medicine used. Generally, higher quality tends reduce the total time until recovery, though mechanics vary. How often a disease can be tended depends on the disease itself.

There are 3 types of diseases to tend to:

The disease will progress over time, and treatment slows down progression. Immunity is gained over time, dependent on Immunity Gain Speed and independent of tend quality. Once immune, the disease will slowly fade. Applies to most fatal diseases, such as the flu, infection, and plague.
The disease does not progress. Treatment must reach a total tend quality (300%) before the disease is cured. Applies to gut worms and muscle parasites.
The disease progresses over time, and treatment slows and even prevents progression. The disease fades only after a certain time. Applies to lung rot, fibrous mechanites, sensory mechanites, and blood rot.

In addition, certain diseases have specialized cures. Infection in a non-vital part can be cured by amputating or replacing a limb, blood rot can be instantly cured with glitterworld medicine, and gut worms can be treated by replacing the stomach.

Immunity gain

Main article: Immunity Gain Speed

Some diseases are a race between the increasing severity of the disease and the increasing immunity to it. This immunity gain is controlled by several factors:

Base immunity gain speed for the particular disease.
InfectionLuck, an uncontrollable generated value that adds randomness to the diseases progression.
The Immunity Gain Speed stat of the patient

Immunity Gain Speed is affected by the following:

Food: Pawns above 12.5% Saturation do not have any penalties to immunity.
Rest: Actively resting gives a bonus to speed. Resting in any bedroll or bed increases the stat, a hospital bed increases it further, and a hospital bed with an attached vitals monitor increases it the most.^[Verify]
Age: As a pawn gets closer to and past their species' life expectancy, immunity gain speed decreases. For baseline humans, immunity gain speed is gradually lost after age 54. Genes that affect the pawns lifespan such as cell instability also affect this progression.
Blood Filtration: Mostly increased via luciferium and slightly increased by a detoxifier kidney
Direct boosts: The Super-immune trait, Immunoenhancer, and genes can increase immunity speed.

Note that these only apply to diseases with immunity mechanics. For example, you don't need to give a pawn with gut worms bedrest.

Disease frequency

Disease frequency by AI storyteller

Disease frequency is controlled directly by the Disease frequency AI storyteller setting.

This value can be changed in custom storyteller settings, but has the following default values by difficulty:

Difficulty	Disease interval multiplier
Peaceful	3
Community builder	2.5
Adventure story	1.3
Strive to survive	1.0
Blood and dust	0.95
Losing is fun	0.9

Disease frequency by biomes

Diseases

Note: Base values of severity and immunity gain speed used in the table.
Treatment's reduction of severity depends on tend quality. 50% tend quality equals half the reduction.

Disease	Description	Severity	Treatment
Infection	Generic infection of individual wounds. Fast moving and dangerous. < 1.25 days to kill if untreated. <1.5 days for immunity. Mild symptoms until extreme, >0.87 severity. (+5% to +12% pain)	Severity: +0.84 / day Immunity: +0.644 / day Treatment: -0.53 severity / day (at 100% quality)	Bed rest and medicine Amputation of infected body part
Flu	Relatively mild and common diseas. Non-elderly adults can survive without treatment, with just bedrest. Lowers consciousness, manipulation, breathing (−5% to −20%).	Severity: +0.84 / day Immunity: +0.644 / day Treatment: -0.53 severity / day (at 100% quality)	Bed rest and medicine
Malaria	Common in tropical biomes. Attacks blood filtration, slowing immunity speed. Later stages cause pain and reduces consciousness.	Severity: +0.249 / day Immunity: +0.238 / day Treatment: -0.077 severity / day (at 100% quality)	Bed rest and medicine
Plague	Fast moving and extremely painful. ~1.5 days to kill if untreated. Constant pain (+20% to +85%). Also reduces consciousness and manipulation (up to −30%).	Severity: +0.666 / day Immunity: +0.522 / day Treatment: -0.362 severity / day (at 100% quality)	Bed rest and medicine
Sleeping sickness	Very slow-moving infection. Native only to rainforests and swamps. Relatively easy to treat if rested. Threat is the amount of rest / medicine required. Mainly lowers consciousness & manipulation (−2% to −20%).	Severity: +0.12 / day Immunity: +0.11 / day Treatment: -0.07 severity / day (at 100% quality)	Bed rest and medicine
Scaria	Drives animals to become manhunters. Exclusive to animals in manhunter packs. Humans and colony animals immune. 100% fatal within 5 days, without cure	No progression	Surgery 3 medicine Medical Skill 8+
Lung rot	Occurs in contact with rot stink. Avoid rot stink to avoid. Resisted by sources of Toxic Environment Resistance. Easily treatable overall. Can drain medicine. Impairs breathing slightly. Minimal symptoms overall.	Severity: +0.300 / day Treatment: -0.700 severity / day (at 100% quality)	Medicine
Gut worms	Doubles hunger rate. Also painful (+20% pain).	No progression Cured at 300% total Tend Quality	Medicine (48hr per tend) Stomach replacement
Muscle parasites	Painful, impairs working. Significant tiredness. Lowered manipulation and movement speed (−30%).	No progression Cured at 300% total Tend Quality	Medicine (48hr per tend)
Fibrous mechanites	Very long-lasting, but improves working. Painful, significant tiredness. Significantly increases manipulation, moving and blood pumping (+50% to all).	Severity: +0.25 / day Treatment: -1.00 severity / day (at 100% quality)	Medicine Prevents severity increase only Resolves from: Min 900,000 ticks (15 in-game days) Max 1,800,000 ticks (30 in-game days)
Sensory mechanites	Very long-lasting, but improves many stats. Painful, significant tiredness. Significantly increases sight, hearing, and talking (+50% to all). Increases manipulation (+30%).	Severity: +0.25 / day Treatment: -1.00 severity / day (at 100% quality)	Medicine Prevents severity increase only Resolves from: Min 900,000 ticks (15 in-game days) Max 1,800,000 ticks (30 in-game days)
Blood rot	Extremely long lasting blood affliction. Quest only. Easily treated, but a drain on medicine.	Severity: +0.40 / day Treatment: -1.00 severity / day (at 100% quality)	Medicine Surgery for immediate cure 10 glitterworld medicine Medical Skill 5+ Resolves from: Min 1,800,000 ticks (30 in-game days) Max 2,400,000 ticks (40 in-game days)
Paralytic abasia	Extremely long lasting. Complete immobility. Quest or transport pod crash only.	No progression 1% per day to instantly cure	Waiting Surgery for immediate cure 10 glitterworld medicine Medical Skill 5+ Always resolves after: Min 1,800,000 ticks (30 in-game days) Max 2,400,000 ticks (40 in-game days)
Infant illness	Inflicted after a sick birth of a baby. 4 different types of illness, from mild to grave. Switches every 1-3 days. Uniquely, reducing severity to 0 cures the disease.	Starts as Major severity (0.33 - 0.77) Severity gain depends on the type From -0.25 to +0.5 severity / day Treatment: -0.35 severity / day (at 100% quality)	Medicine Resolves from: Min 480,000 ticks (8 in-game days) Max 600,000 ticks (10 in-game days)