Spreading plagues


Rules Questions


It seems that the spread of diseases in only implied in the rules (I can't find a definition of how Inhaled diseases spread for example). Does anyone have some reasonable rules for determining how fast a disease will spread once a few people have been infected with a disease and mingle with a larger group?


The rules really only cover a PCs chances of becoming infected so that the GM knows whether or not to give them the affliction. Outside of that...the disease grows at the rate the GM needs or wants it to.

I can't find information on infectious disease transmission rates from medieval times to really give a realistic starting point.


Claxon wrote:

The rules really only cover a PCs chances of becoming infected so that the GM knows whether or not to give them the affliction. Outside of that...the disease grows at the rate the GM needs or wants it to.

I can't find information on infectious disease transmission rates from medieval times to really give a realistic starting point.

It would be approximate to disease transmission rates in the late 1800s and early 1900s. Our understanding of medicine was only marginally better then than it was in the midieval times. Nothing like the leaps and bounds we have today.


JohnHawkins wrote:
It seems that the spread of diseases in only implied in the rules (I can't find a definition of how Inhaled diseases spread for example). Does anyone have some reasonable rules for determining how fast a disease will spread once a few people have been infected with a disease and mingle with a larger group?

I would assume transmission would depend on factors such as how the disease is passed (airborne, simple physical contact, transmission of bodily fluids, etc.) the virulence of the pathogen in question, normally I assume this is represented by the saving throw required, the population density of the area the infected is in and the amount of ambulatory time the infected has before they realize they are sick and before the illness renders them immobile or dead.

Assuming most people in the world are level 1 commoners you can get a rough guess of their chances to actually contract something once exposed based on their save bonus versus the DC of the pathogen in question.

The hard part will be estimating rates exposure based on transmission form of the disease, population density of the region the infected are in, etc.

Assume that airborne transmission spreads much faster than simple contact and contact much faster than swapping of fluids. If the diseased travels through high population centers during peak day times and encounters a lot of potential infectee's then the contagion can spread VERY quickly if airborne or contact based. If the area has very poor sanitation and rats/ticks/vermin are an issue then diseases that transfer by bodily fluids are more dangerous in those area's due to cross transmission by ticks, mites, mosquitoes, etc.

As a totally non scientific set of rules:

Airborn transmission = 40% chance to make a save if you are within a 20 feet of someone infected.

Contact transmission = 30% Chance to make a save if you are touched briefly by the infected.

Transmitted by bodily fluid = 20% chance to make a save for surface contact (this assumes 2 people are wounded and make some form of contact like someone laying on hands, recieving a touch spell, hand to hand combat, etc). Increase the chance to 40% if intimate contact happens or if either or both parties are severly wounded (say below 50% hit point).

Have poor sanitation increase the fluid and contact transmission rate by 5%.

Make the number of checks made based on population size of the area the infected is in. The more people contacted and exposed, the more it can spread.

Thorpe 1d4 people exposed per day
Hamlet 1d8
Village 2d8
Small town 4d6
Large town 5d10
Small city 20D10
Large city 40D100
Metropolis 60D100

So make your roll for population of potential infected, make the appropriate number of saving throws. Then the next day, multiply the result of the potential infected roll by the number of actual infected in that region that are still mobile and unaware of their infection (disease onset should tell you when they become aware or immobile).

Give a flat 25% chance that someone from the infected pool travels to a nearby population center per day per infected to represent the possibility of a traveler being infected and moving on. Caravan merchents and their guards, traveling circuses or minstrels/players, adventurers, etc.

OR

TLDR - You could make up anything you need for the story and it would work.

Mind you that depending on the power of the clergy in your game world, diseased based CURSES may spread better than actual diseases, as they are harder to counter.


Considering the average person is probably a 1st or 2nd level Commoner or Expert with a max Fort save of +1, and most diseases have DC in the mid teens to twenties, I would say the spread of disease is very fast.

It depends on information we aren't always given (at what point is the subject contagious... during onset?... until a save is made?... until cured?).

And, of course, how much exposure the subject gets to other people. Spread rates will be higher in urban areas than less densely populated areas.

But imagine that one person with Final Rest enters a city. The DC 16 means most people have a 70% of failing the Fort save if exposed to him while he is contagious.

This helps explain why plagues still are an issue in a world with magic. There just isn't enough magic to cure everyone, and the disease can quickly out spread the ability to cure it.


X=average chance of NPC failing the infection save
Y=average number of "infectious" contacts a day
P=infectious population
For the extra simplified version we start with patient zero (P=1) and multiply to figure out the number of new cases (so X*Y) and add that to P. Then we subtract the number of patients who are now presenting symptoms (depends on disease). Then you just iterate as long as you like. That gives us a basic model of people interacting normally unless someone is sick, then they shun them. Honestly Y should decrease as P increases, not all populations have the same save (warriors are better off than commoners or experts), richer people might chug anti-plague before going out, lots of other variables. But the simple version is just multiplying "how often do they interact with people in a way that might infect them" and "how often do they fail the save and catch the disease".


Actually Samasboy, your points are exactly why the transmission rates of disease would have to be fairly low on Golarion.

Otherwise, by now everyone would have died from disease long ago.

The lowest save DC is a 12. But requires you to make 2 consecutive successful saves against that DC. Commoners and other NPC classes except for warrior have a +0 unless they are 3rd level or higher.

That means the probably of actually successfully saving against the disease two times consecutively is....16% if I've done my math right for the majority of the population. That will result in a incredibly high mortality rate with people eventually succumbing to the effects of the disease. In order to mitigate that I would guess the actual transmission rates would need to be very low.


Claxon wrote:

Actually Samasboy, your points are exactly why the transmission rates of disease would have to be fairly low on Golarion.

Otherwise, by now everyone would have died from disease long ago.

The lowest save DC is a 12. But requires you to make 2 consecutive successful saves against that DC. Commoners and other NPC classes except for warrior have a +0 unless they are 3rd level or higher.

That means the probably of actually successfully saving against the disease two times consecutively is....16% if I've done my math right for the majority of the population. That will result in a incredibly high mortality rate with people eventually succumbing to the effects of the disease. In order to mitigate that I would guess the actual transmission rates would need to be very low.

That also is mortality assuming 0 outside help. There is possible treatment from the Heal skill as well before magic comes into play.


I also think that any hint of the spread of a disease that lethal would be tracked down and controlled by clerics of many different stripes.


Claxon wrote:

Actually Samasboy, your points are exactly why the transmission rates of disease would have to be fairly low on Golarion.

Otherwise, by now everyone would have died from disease long ago.

The lowest save DC is a 12. But requires you to make 2 consecutive successful saves against that DC. Commoners and other NPC classes except for warrior have a +0 unless they are 3rd level or higher.

That means the probably of actually successfully saving against the disease two times consecutively is....16% if I've done my math right for the majority of the population. That will result in a incredibly high mortality rate with people eventually succumbing to the effects of the disease. In order to mitigate that I would guess the actual transmission rates would need to be very low.

But you are failing to factor in some things.

Using Final Rest as our example again, the onset time is a week.

Is he contagious during the onset time? We don't know, but many real world illnesses are transferable before symptoms begin appearing.

So that's a week to spread the disease to 70% of people you come into contact with. That is potentially a staggering number of new subjects.

Then it takes you about 3 or 4 days to die (and yes, most people WILL die). Still plenty of time to infect a bunch of people.

Most diseases don't give us information on how far they can infect new people. This is mostly only for inhaled diseases, but for example Blightburn is considered Contact but contact is defined as 60'. Real world diseases spread by body fluid particulate (coughs and sneezes) can spread 12-15 feet and the microbes can survive for hours.

Some disease are so deadly that they will burn out before spreading too much. But, especially in a city, many will spread very quickly. Most will have a hard time spreading from one population center to another, as the carrier will die before arriving at the next location.


That is true gilfalas. If an individual has access to someone with the heal skill that can beat the save DC of the disease then that person gets a +4 to their save.

That means that the person still only has a 36% chance of successfully saving against the DC 12 disease. Magical access to healing is probably too scarce for it to affect the over all numbers too much. Select individuals will be safe, but there would be too many people for clerics to heal people effectively, especially since it would require at least level 5. We would need better demographics about levels and classes for Golarion to extrapolate more accurately.

Samasboy1 wrote:
Claxon wrote:

Actually Samasboy, your points are exactly why the transmission rates of disease would have to be fairly low on Golarion.

Otherwise, by now everyone would have died from disease long ago.

The lowest save DC is a 12. But requires you to make 2 consecutive successful saves against that DC. Commoners and other NPC classes except for warrior have a +0 unless they are 3rd level or higher.

That means the probably of actually successfully saving against the disease two times consecutively is....16% if I've done my math right for the majority of the population. That will result in a incredibly high mortality rate with people eventually succumbing to the effects of the disease. In order to mitigate that I would guess the actual transmission rates would need to be very low.

But you are failing to factor in some things.

Using Final Rest as our example again, the onset time is a week.

Is he contagious during the onset time? We don't know, but many real world illnesses are transferable before symptoms begin appearing.

So that's a week to spread the disease to 70% of people you come into contact with. That is potentially a staggering number of new subjects.

Then it takes you about 3 or 4 days to die (and yes, most people WILL die).

Some disease are so deadly that they will burn out before spreading too much. But, especially in a city, many will spread very quickly. Most will have a hard time spreading from one population center to another, as the carrier will die before arriving at the next location.

This actually just furthers my point I made to you. Transmittance rates must be incredibly low on Golarion, or else the world would have died long ago to disease. The only way this doesn't happen is if settlements are literally wiped out and any travelers are killed by the disease before the can reach other substantial settlements.


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Claxon wrote:
This actually just furthers my point I made to you. Transmittance rates must be incredibly low on Golarion, or else the world would have died long ago to disease. The only way this doesn't happen is if settlements are literally wiped out and any travelers are killed by the disease before the can reach other substantial settlements.

Also remember that even in the real world, you could be touched by someone with an infectious disease or be in the area of an air spread one and NOT catch it or even be infected.

I tend to look at saving throws as what happens once the disease is definitely on/in you. Not all exposures will result in a situation where someone even gets infected to the point a save is required.

Plus as usual we should all remember that Pathfinder was not written as an accurate biological apocalypse simulation. It is game rules meant to try and make diseases scary to the adventurer.


Thanks for all the thoughts.
If I was dealing with a generic outbreak of plague I would be entirely happy with me deciding on an arbitrary basis who gets ill and who does not among the vast crowd of NPC's and would look at historic disease outbreaks to get an idea of the range of possibilities.
In this case my players will be introducing disease into a small group of people by deliberately infecting a small number of them so it would be useful to have a system for working out which of the other members of the garrison will be exposed to the disease without making it GM fiat.

In the area there is precisely 1 cleric of over 5th level so access to remove disease is somewhat limited , although he will try to solve the problem and it will be a boon to the pc's if all of the clerics 3rd level spell slots are remove disease rather than combat useful spells. There is also a Paladin not sure if he can remove disease via lay on hands if he can that can probably end the disease


Claxon wrote:
The only way this doesn't happen is if settlements are literally wiped out and any travelers are killed by the disease before the can reach other substantial settlements

Which is exactly what is described in Golarion lore in multiple instances. Irrisen, Ustalav, Iobaria...

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