There are a variety of diseases present in the world, with varying rarity, infectiousness and virulence.
Diseases are infectious, and so contact with a diseased person can result in contracting the disease. The degree of infectiousness depends on the disease - when a character comes into contact with a disease, roll a D to see whether he/she becomes infected. A mildly infectious disease will infect the character on a 1-2 roll, a normally infectious disease will infect on a 1-4, and a highly infectious disease will infect on a 1-5.
If a character is infected with a disease, there is normally an incubation period, while the disease attacks the character's body and before the first symptoms manifest. This is normally a few days. During this period, the character will be infectious, and any other characters coming into contact with the incubating character will need to make infection rolls to see whether they become infected.
Once the incubation period is over, the first symptoms of the disease appear. The symptoms will vary based on the disease, but may include fever, nausea, inflammations, etc. Treat a character manifesting these early symptoms as having a Light wound from the disease.
Diseases vary in virulence, from minor diseases (such as colds), through normal diseases (such as influenza), to lethal diseases (such as bubonic plague or black death). Every 12 hours after symptoms first manifest, an infected character must make a dice roll using his/her ST (but no skill, treat as skill score of zero). There is a bonus of +3 on the dice roll if the character is resting for the whole period, and a penalty of -3 on the dice roll if the character is exerting him/herself strenuously during the period. There is also a bonus on the dice roll if the character's disease is being treated (see below). The dice roll result determines whether the disease gets better or worse, as follows:
| Virulence | Increases by one wound level (i.e. gets worse) | Stays the same | Decreases by one wound level (i.e. gets better) |
| Minor | -4 or lower | -3 1/2 to -1/2 | 0 or more |
| Normal | 0 or lower | 1/2 to 4 1/2 | 5 or more |
| Lethal | 4 or lower | 4 1/2 to 9 1/2 | 10 or more |
When the disease level is reduced to Normal (i.e. decreased by one level from Scratch), the character is fully recovered from the disease. If the disease level reaches Fatal, the character dies.
Once a character has had (and fully recovered from) some diseases, he/she will be immune from them. For other diseases, they are chronic, and the character can become reinfected with the disease many times.
A disease wound is not subject to infection or shock. It does, however, count towards ongoing impairment, which means that the more ill a character gets, the worse his/her chance of recovering.
When a person is ill, a character can diagnose what is wrong with that person (i.e. what illness they are suffering from) if the character makes a dice roll using his/her Medicine (including diagnosing disease specialism) + PE.
The difficulty depends on how common the disease is, and how many symptoms the person is manifesting, as follows:
| Level of disease | Common disease | Uncommon disease | Rare disease |
| Incubation period | 5 | 7 | 10 |
| Light wound | 3 | 5 | 8 |
| Medium wound | 2 | 4 | 7 |
| Serious wound | 1 | 3 | 6 |
| Critical wound | 0 | 2 | 5 |
| Fatal wound (i.e. diagnosing from dead body) | 0 | 1 | 4 |
A correct diagnosis (i.e. successful dice roll) will allow the character to treat the disease (and hopefully help the ill person recover from the disease).
If a character has successfully diagnosed an ill person's disease, he/she can attempt to treat the person's disease. He/she should make a dice roll using his/her Medicine (including Treating disease specialism) + IQ.
The ill person may then add the carer's dice roll result as a modifier to the ill person's dice roll to determine whether the disease gets better or worse (see above).