Avoiding Ninja Cholera

On the off-chance the OP or anyone else sees this, I'm actually working on reverse engineering this game (thanks, quarantine) and can answer this for certain.

In the oregonii.dat file, every health problem has a flag determining whether it's possible to die instantly, and a separate variable for the chance of that happening if you get that problem (i.e., if you do contract the illness/get injured, do you get the normal set of choices, or do you just die?)

Those diseases and chances are:

• Accidental gunshot, 20%
• Animal bite, 10%
• Animal mauling, 10%
• Cholera, 20%
• Drowning, 25%
• Freezing, 10%
• Internal injuries, 20%
• Snakebite, 5%

If you die instantly, the death screen has a custom message. If you go into the oregonii.eng file, in the list of strings ("____ has cholera," etc.) for any given disease, it's the last one. (Incidentally, there are a couple of these messages that are definitely customized but seem unused, so maybe at one point there were other things you could die instantly from?)

As for how to avoid it, unless there's some other factor I haven't found -- medical skills don't seem to matter for this specific roll -- all you can do is edit the .dat in a hex editor. (Which means you can also do stuff like make a bad cold instantly fatal 100% of the time.)