Our campaign is about a Plague that is infecting a cosmopolitan city.
Our Elf pc has a belief about caring for the health and well being of his human friend, a Knight.
Elves are immune to disease because of Essence of Earth.
Now to challenge the Elf belief it would be cool to have the Human PC be infected by the Plague. In fact it adds verisimilitude to the plague if the Human PC is as susceptible to the plague as every other poor soul in the city.
How do I do this?
Roll a die of fate? Make a health test against an Ob?
How would you design the effects of the plague mechanically?
Two ways to go about it in my view, but these both assume mundane, person to person transmission and nothing like it literally being a magical curse that strikes people at random (or even something more mundane like being insect transmitted)
Simply spending too much time around the infected, no matter what, risks an exposure, make a health test to avoid getting the illness.
Add contracting the illness as a complication of failure when dealing with the infected, this can even be in addition to other failure consequences. In this case, you might allow a health test to avoid symptoms, but I would probably still make them a carrier in this case. And I don’t think I need to tell anyone in the current year how dangerous that could be.
As for the disease itself, I would simulate it as obstacle penalties related to specific symptoms, and maybe injuries to represent an overall general weakness.
If the disease is recoverable on one’s on, just make health tests to see how bad symptoms get/if they get better, you could even do it in one with let it ride “If you make this you only have minor symptoms and recover in a week, if you fail it will take longer and the symptoms will be worse”. Medical attention can give bonus dice to the health test.
If you can’t recover without treatment then medical tests are required to make the health test at all.
Finally, if there is nothing the current medical knowledge can do, use health and medical tests to see if the character progresses to the next stage of the disease or manages to remain stable, with the ob likely going up over time if such a thing is inevitable.