A look at disease in T2k v2.2

[Vespers War]

I decided to look into v2.2’s disease rules to see how they could be adapted to model chemical or biological weapons. Just as a start, here’s an explanation of the rules with some probabilities worked out.

If a character is exposed to a disease, they make an Average: Constitution throw to avoid infection, subtracting any fatigue and the disease’s infection number from their asset. Since attributes average a smidgen higher than 5, the average character starts at a 10 for an Average: Con throw. Infection numbers for diseases listed in the v2.2 book range from 3 to 6, so an average character is looking to roll somewhere between a 7 or less to a 4 or less to avoid infection if fatigue is 0.

Recovery requires a d10 roll above the disease’s Recovery Number, which varies from 18 to 26. Bonuses are for treatment, for a medic’s Medical (Diagnosis) skill, and for Constitution. Minuses are a -1 each for inadequate food and inadequate shelter.
Subtracting the average Con of 5 means the effective target number on the recovery die is 13 to 21.
The average roll with no treatment and an average Con will be 10 (5 on the die and 5 Con). Treatments add +1 to +4 individually, and diseases range from +2 to +8 for the effect of all of their treatments being applied correctly. In general, diseases with higher Recovery Numbers have higher treatment bonuses. Targets will range from 9 to 14 after subtracting the maximum treatment (and Con) from each disease’s recovery number.
On the medical side, a Nurse or Doctor will add at least +3 – Nurses have a minimum Medical (Diagnosis) 3 and Doctors will have a minimum Medical 7 between Medical School and the first term as a Doctor, but since it won’t necessarily be in Diagnosis they might only get half their skill (rounded down).
This means with a professionally trained but minimally experienced healer, another 3 points come off the target and the roll needs to be 6 to 11. The highest numbers are still impossible on a d10, but each roll has 40% odds for the weakest diseases.
If treatment is started during Phase I of the disease, two rolls are allowed. If it starts in Phase II or there is no treatment, only one roll is allowed.
If all the rolls fail, the player rolls a d10 against the Failed Recovery Death Probability. A roll of its number or lower means death, while above the number means recovery. About half the diseases are 1 (10% chance of death on a failed recovery), while the ones that are higher are cholera and typhus (2), food poisoning (5), and rabies and plague (10).
Most recoveries include post-recovery symptoms of 1-3 levels of fatigue for 1-20 weeks.

For an average character, it is incredibly easy to catch a disease (65-80% chance for Con 5 with 0 fatigue), the recovery number is generally decent if you get good medical treatment, getting diagnosed and treated early gives two rolls and makes recovery much more likely, and most diseases are not highly lethal.

The most dangerous disease is pneumonic plague, which has Infection 6, only +4 Treatment, Recovery 23, and Death Probability 10. A Con 5 character exposed to that has an 80% chance of infection and needs correct treatment starting in Phase I, adequate food and shelter, and a doctor with Medical (Diagnosis) 5 just to have a 10% chance of making each recovery roll, which gives a 19% chance of making at least one roll. Failing both recovery rolls mean certain death, and failing to start treatment until Phase II means you need someone with Diagnosis 7 to have a 10% chance of survival. Not getting treatment at all raises that requirement to Diagnosis 9 for a 10% chance of survival.

The least dangerous other than Minor Disease is probably dysentery, which is Infection 3, +4 Treatment but with a lower Recovery 19, and Death Probability 1. The only disease with a lower post-treatment recovery is Cholera (+7 Treatment against Recovery 21), and it’s both more infectious (5) and more lethal (2). Your typical Con 5 character with a trained-but-inexperienced medic has a 65% chance of infection with dysentery and two recovery rolls with a 15% chance of success each (27% chance of at least one success), with only a 10% chance of death if both fail. Against cholera they'd have a 75% chance of infection, 20% chance of success on each recovery roll (36% chance of at least one success), and 20% chance of death on failed recovery.

Overall, a high Con and a doctor with high Medical (Diagnosis) are going to be the two biggest boosts to survival rates, since someone with Con 10 will only have a 15-30% chance of infection (depending on disease) and high Diagnosis helps with starting treatment quickly, while both improve recovery rolls directly.

I'm not sure when I'll get around to fiddling with weapons that work like diseases, but this at least gives an idea of how some of the probabilities work within the disease system.