A tiny mod, containing only one text file, that introduces three plagues and rabies for fortress and adventurer mode. Use for your enjoyment or whatever you like, including inclusion in your own mod without credit if you want to - I just wanted to add new functionality to the game. Should be compatible with just about anything.

Summary:

Deities occasionally curse intelligent creatures to become plaguebearers. These otherwise unremarkable poor souls then migrate to and spread plague in your fort. Plague victims experience different effects over time, but at this time will never completely recover from the plague. Plague sufferers can infect one dwarf per day to share the same fate. They will also infect your enemies and can be weaponized, as those enemies then infect other enemies. Unintelligent creatures can get rabies instead.

Currently includes the following:
* bubonic plague - despite how it sounds, mostly harmless -- just blisters, fevers, vomiting blood and dizziness
* contagious manic depression - makes dwarves periodically emotionless and drowsy, mischievious or occasionally berserk
* blackheart plague - the money plague, causing oozing, nausea, blindness, random rotting of body parts and limbs over time and pain intense enough that dwarves can pass out
* rabies - an animal becomes contagious to both dwarves and other animals after three weeks, and permanently berserk at four weeks. A dwarf becomes berserk and contagious at four weeks and dies at five weeks.

DFFD: https://dffd.bay12games.com/file.php?id=7755

A more fun mode where EVERY dwarf is a potential plague spreader instead. This is done by modding creature raws so that dwarves all have a 1% chance to turn into a random type of plaguebearer upon entering the map. Note that if you do not add this, plagues may not exist in your particular world at all due to divine laziness:

DFFD: https://dffd.bay12games.com/file.php?id=7756

Tested in arena mode to make sure syndromes and interactions work correctly, ie. plagues exist, spread and do bad things. Not tested in fortress mode for balance at all, but from arena mode testing it seems plagues get very serious very fast if not contained.

If anyone wants it, I can also very easily make "medicine" that can remove plagues based on this. For anyone who's interested, just add IT_CANNOT_HAVE_SYNDROME_CLASS:MEDICINE to all the plague interactions and SYN_CLASS:MEDICINE to all your medicines.

Some notes on how to modify this to your liking or apply this to your mod:
* To make milder and more severe plagues, just edit the syndrome tags that follow any plague at interaction: DISEASE_PLAGUE*
* To make new plagues that spread similarly, just copy a plague and change the plague number and the syndrome tags. They will work similarly to these. If you want to use the add on mod that makes plagues more common with them, the files are annotated so as to how to do that. Basically just add more options with your plague numbers and probabilities to PLAGUE_SWITCH
* To modify how plagues spread, modify the SPREAD_PLAGUE interactions. If you want them to not spread as aggressively, add a higher amount of time to the first [CE_CAN_DO_INTERACTION:START:1200] bracket (replacing 1200 with eg. 3600 for 3 days) and the immediately following [CDI:WAIT_PERIOD:1200] tags both. Adding to only the first one results in a higher latency period, but plagues spread very fast once started. Adding to only the latter one tends to create a domino effect, because while dwarves spread plague slower after they've started, they can still spread it to one other dwarf very fast if you didn't edit the first tag.

No offense to anybody affected by a psychiatric disorder intended - the colorful vocabulary is supposed to reflect dwarven concepts of mental illness, not ours.

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Adventurer mode note: while people you meet suffer from plagues and you yourself can be infected in adventurer mode if you go near them, the current nature of interactions means that unlike everyone else, you as a player are not actually forced to experience the symptoms unless you choose to, and you will spread plague consciously only, not automatically. This can't be fixed at this time since there's no way to my knowledge to make you use the interactions - a partial fix is possible by changing the interactions to syndromes but this would break the randomization of symptoms over time of these plagues in fortress mode. Sorry.