Disease Pools
To determine what diseases a culture starts with, disease pools will have to be established first. There would be 12 disease pools. Distribution would begin at the tropics. Each tropical pool would get 2 jungle diseases and 1 water-borne disease. Each of the temperate pools would get 1 of the 2 remaining water-borne diseases and 2 other diseases. Then the arctic pools would get 2 diseases from the remaining 5 and would share the last one. Then each culture would get it's number of initial diseases from it's disease pool. If a culture is in 2 or more disease pools, it will select from the pools it was in.
Here is an example of what one game's disease pools might look like:

sub/arctic 1(Influenza, Bubonic Plague, Typhus) 7(Whooping Cough, Leprosy, Measles)
temperate 2(Diphtheria, Hantavirus, Measles) 8(Tuberculosis, Syphilis, Influenza)
sub/tropical 3(Malaria, Yellow Fever, Cholera) 9(Yellow Fever, Dengue Fever, Diphtheria)
sub/tropical 4(Sleeping Sickness, Dengue Fever, Dysentery)10(Sleeping Sickness, Malaria, Cholera)
temperate 5(Tuberculosis, Smallpox, Leprosy) 11(Dysentery, Bubonic Plague,Smallpox)
sub/arctic 6(Syphilis, Whooping Cough, Typhus) 12(Typhus, Hantavirus, Measles)

Initial Diseases

Initial Diseases will represent those the people have previously been exposed to. Every province of every Civ. and barbarian "Culture" in the game will start with 1-3 diseases from the 19 modeled in the game. This assumes that each different cluster of any barbarian culture will be treated as a different province (ex. If culture #17 is divided by Roman territory, each half would be treated as a different province). By modeling every province rather than the Civ/Culture as a whole we can allow for greater diversity and realism in the system with only a few extra calculations.
To calculate the number of initial diseases a civ has, we would generate a number (1-10) then modify it by population density and climate. Climate would modify the number based on the zone the Province is located in:

Climate Zone{Modifier(+/-)}[#of DiseasePools]
Sub-arctic/Arctic {3} [4 pools]
sub-arctic/Temperate {2}(a culture would use this modifier if it overlaped both zones listed)
Temperate {1} [4 pools]
Temperate/subtropical {-2} (a culture would use this modifier if it overlaped both zones listed)
Subtropical/Tropical {-3} [4 pools]

Once modified for climate zone and density, the number of diseases is as follows:

Modified Number /Number of Diseases
1 or less / 3 Diseases
2-4 / 2 Diseases
5 or more / 1 Disease

Disease Table
Disease / Mortality rate / Str. Increase
Bubonic Plague B /15-30%/ 2
^Cholera B /10-20%/ 3
^Dysentery B /10-20%/ 5
$Dengue Fever V /10-20%/ 4
^Diphtheria B /5-10%/ 5
$Ebola V /15-30%/ 2
Hantavirus V /15-30%/ 3
$HIV/AIDS V /1-5%/ 4
Influenza V /1-20%/ 4
Leprosy B /1-2%/ 5
$Malaria V /5-10%/ 3
Measles V /1-5%/ 5
$Sleeping Sickness V /5-10%/ 3
Smallpox V /5-10%/ 4
Syphilis B /1-5%/ 5
^Tuberculosis B /5-10%/ 3
Typhus B /5-10%/ 3
Whooping Cough V /1-5%/ 4
$Yellow Fever V /5-10%/ 4

^ = water-borne;
$ = jungle;
* = Ebola and HIV/AIDS are never initial diseases but come in later in the game to preserve some history to disease.
V = viral
B = bacterial

How Disease Works
For every disease a culture has it will also have a resistance number for that disease. Every disease will also have a strength(str.). It is by comparing these two numbers we will determine what effects a disease will have:
-If disease str. is less than the culture's resistance there is no effect.
-If disease str. is equal to or greater than the culture's resistance, a "check" is made. (A check is a random number, 1-100, that is generated for str. then str. is subtracted from it; the same is done for the resistance. The lower number is the winner.)
An example of how the check works:
---Disease str. is 35 and Resistance is 30; a 15 is generated for str. and a 36 for Resistance; after subtracting str. from 15 you get -20 and 36-30 is 6, so the disease "wins" the check and has it's effect.
-If the resistance wins the disease has no effect.
-If the disease wins there is an epidemic, and a loss of population equal to the mortality rate of the disease. This loss is for one turn only! When an epidemic occurs, resistance increases by 1-5
-If it is a tie, then nothing happens and a check is made again next turn.

Some other things to keep in mind:

-Base resistance is 25 for all diseases, except initial diseases, which is 50.
-Base disease str. is 15-25 (Regardless of what str. it is at when transmitted to a new culture, a new str. will be determined. However, the range on it will be the difference between the numbers above and actual disease str. For example, if transmitted at str. 55 the disease str. for the new culture would be 40-30, or 30-40.)
-Disease str. and resistance can never exceed 100 (except if strains are used, then disease str. can reach 110, but only during flare-ups.)
-Disease str. increases by 1 every number of turns listed on the chart.

Spreading Disease
Whenever any kind of contact is made between two civs/cultures (merchants, armies, migrations, etc.) there would be a chance, equal to the home culture's resistance, of the disease spreading. Also when a civ encounters jungle, there would be a chance (equal to the disease str. in that region) of them catching a jungle disease and possibly bringing it back home.
Ebola and Aids will have a 25% chance of infecting anyone who travels in the jungle, however only after whatever turn the year 1900 occurs on.
Note: When Migrations occur, add +1% per head, or fraction thereof, for the percentage chance of disease spreading.

Modifiers to Resistance/Mortality Rate

-improvements: temples (of religions that care for sick), hospitals, etc. will each have individual effects
-famine will reduce resistance temporarily, while famine continues, by -3 per turn (cumulative).
-technology- some will have individual effects on diseases, chance of spreading or Mortality rates (like vaccinations will cure some diseases).
-overcrowding, -3 for every province with 1,000,000 of population, and an additional -1 for every 500,000 afterwards.

Strains
*This is just an idea I'd like to get some feedback on though it may be just a bit too complicated for the game*

Each disease will have several strains active until sufficient contact has turned it into a single strain. An example would best explain this:
The Greeks have smallpox (strain one) and the Egyptians have smallpox also (but strain two). Upon contact & transfer of both strains between the 2 civs, there would be a flare-up (+10 to disease str. for one turn) of the disease as the two disease pools collide and meet new cultures. Then these two strains would be annialated and would become strain three (a new strain) within both civs.
Now if the Greeks had given the Romans strain one previously but no longer had contact with them, strain one would survive with the Romans but the Greeks and Egyptians would have strain three (strain two would be annialated). But if the Greeks again contacted the Romans and transmitted strain three, and they still had good contact with the Egyptians, all three would then have strain four, and strains one, two and three would have been annialated.
Strains would never be seen by the player, they would only help simulate flare-ups of disease as disease pools collide. They would also simulate a decrease in flare-ups as sufficient contacts are made between cultures thereby increasing the size of the disease pools.

Agricultural Disease

Here's what i have to say sofar (this is before anything on agricultural disease is mentioned):

First artic zones would have fewer diseases and tropical more because baterica thrives in the warmer climates. Also any culture coming from an artic area would carry few diseases (Native Americans) since most who surived the journey wouldn't have been very sick.

Also later on we should include genetically engineered diseases. Although never used outside labs, if they did so the results could be deadly.

Also the invention of medicine will temporarily raise the resitance rate, however it will create strains of super-diseases in the long run.

I think strains are a good idea. One thing though, is over time, when new powerful medicines are created and through genetic engineering should also create new strains. Also a new strain is almost always more powerful than the previous one. Another thing is using your example of the Greeks, Romans and egyptians that if the greeks transmitted strain one to the romans after X turns the disease would mutate to strain four at the same time the strain 1 and 2 are combining to form strain 3. This would solve the problem IMO of the continuous creation of new strains for most of history. In later periods new strains could radomly occur do to the constant mixing of cultures.

Genetically engineered diseases I thought about, I was just thinking we leave some "extra" disease slots and let the player have the option of engineering one or not. These would start with a higher base str. than the "normal" diseases.

Super-diseases could be created with the evolving disease str. Maybe we could have a tech called innoculation for bacteria diseases and vaccination for viral. After it is discovered then either double the evolution of the disease rate or add a de-evoulution rate to the Resistance number--I had forgotten to include that.

And for the strains Idea, I couldn't come up with anything to stop the continuous creation from getting out of control but your idea is great. As far as new strains appearing after using powerful Medicines, maybe a random culture's strain would mutate. For example let's say the English had smallpox strain 7, a few turns after the discovery of vaccination it mutates to strain 8(which noone else has).

I think your model is very good!
My questions are:
1) Isn't it too expensive to check all cultures for all diseases every turn? Any chance of making it cheaper in computational terms?
2) Why disease str is lowered if resistence wins? That culture may have won, but if a new culture enters the scene, the disease should be as mortal as always, so I think disease str should never change really IMO

Great Job! I agree with you that on a provincial basis is probably the best way to handle diseases. I'm really crushed for time tonight, so I will just raise the usual difficult comments!

I didn't see population density in there in terms of allocating the number (and also strength) of diseases. Did I miss it? If it's not there, I think it Really needs to be there.

Your method for figuring out if there is an epidemic seems flawed to me. Here's my difficulty:
When the resistance is sufficient to keep the disease in check, there is a 0% chance of epidemic. As soon as the Disease Is A Single Point above the resistance, then there is a 50% chance of an epidemic (well, actually like 51%) Boy that's a rapid turn on in the likelihood of disease! I think you instead need to use something like a percentage chance that is twice the advantage of the disease over resistance.

In terms of diseases spreading, all contacts should not be equal. A huge army tromping around is much more likely to infect local inhabitants than a few lousy merchants.

Finally, I think there needs to be some possibility of geographic separation preventing a disease from showing up, say in the New World. In other words, the temperate zone of the New World would not necessarily have the same diseases in it as the temperate zone of the old world. For the real world we could use the historically correct ones, and for artificial worlds just make some arbitrary choice to segregate as appropriate.

Good Work!

As for the 50% thing, I'm not too sure of any other way to handle it (that's why I finally brought it online, I was running out of ideas). The thing about using twice the advantage of disease over resistance would seem, to me, too low. Most of the time the disease will only get a few points above resistance anyway, so I don't think we should use such a low number or disease would just be a nuissance, not the real threat it is.

The various levels of disease str vs. resistance. FE Europe was highly resistant to smallpox and although people died from it when it was epidemic, a lot didn't. However when it was brought to the americas because no one had ever had smallpox before very few survived. The strain wasn't any more powerful, it was just the resistance level that was.

Also there are several levels for how easily trasmittable a disease is. Here's the ones i remember. Higher the number, the more easily transmitable

Class 1
Long Term Relationship (FE Tuberculosis)

Class 2
Ijestion of Previous Host

Class 3
Sexually Transmitted

Class 4
Contact
Water Borne

I'm with LGJ on the smallpox point. In fact, I think I've come up with a way to simplify your strains idea, make me happier on the chance of an epidemic happening, and tying in population density. Here it is...

First of all, let's say we don't cap the value of the disease or strain at 100,but let it go arbitrarily high. The disease grows in strength when it has an ideal breeding ground. Usually that will mean high population densities and unsanitary conditions, etc.. So an existing disease will never give a province that has developed adequate resistance problems, unless the population, or level of sanitation changes dramatically for the worst. However, when those things do happen (or other things that generate new strains of disease) and then disease grows in strength by a TBD amount. So I think this takes care of the need to have strains, all you need to do is keep track of the strength of the disease in each province.

Secondly, your case where just a smidgen more of disease can result in a 50% chance of an epidemic really bothers me still. How about if, we double my previous suggestion, so that the chance of an epidemic occurs is now four times the difference between the disease strength, and the local resistance. I think this will still make epidemics clearly likely to happen, and almost certain for the classic case when local resistance is low to a new disease. Also, because we're using the difference, we are now free to let disease is have strengths up to an arbitrary level, because resistance (augmented by sanitation and medicine) can also get very large. So we can in principle model things like antibiotic resistance, although I suspect we should never get quite that detailed!

I really think this model would generate enough disease to keep most of the purists happy. Perhaps too much, but we can always tone down things that cast too big a shadow on the game.

Mark, I agree that the way I handled disease str. vs. resistance isn't that good, and capping off at 100 doesn't really need to be there, and that yours is much better, I'll change it. But I'm with LGJ that the strains should still be included.

Ok you start out with X strains. Each with differnt strengths. Well just because a culture is resistant to 1 strain doesn't ness make it resistant to another. Influensa in a good example because every year people have to get new shots for all the new strains that developed because they may not be resistant to the new ones, but still be so for the older ones.

I also didn't see anything about a total lack of immunity when new contact between civs is made (as in old world meets new world diseases). Mortality rates in these cases have been known to be as high as 98%. This might ruin the game from a player's standpoint, though.

Epidemics that occur in pops with descent medicine, etc. may still run rampant. The influenza epidemic at the end of WWI in the U.S. killed 21 million people (although I don't think all the resources of the nation went to help these people, generally poor immigrants). Wiil you have random events in Clash, that this could fall under? Or would this also be too much for the average player?

Diseases are also subject to natural selection. One that is too weak will not establish itself in a host body, while one that is too powerful will kill its host too quickly for it to pass to another host, and not become established in a pop. Eventually any given disease tends to move towards the middle, allowing us to live long enough to pass our infection along to another human. I feel that this is what you're trying to accomplish with the "strains", but IMHO there should be an easier way to model diseases establishing themselves permanently in a popualtion w/o excess details.

Lastly, I have a disease model in the civ3 forums (under Miscellaneous/other in the "List")that you can feel free to look at. It needs updating, though. And read "Guns, Germs, and Steel" by Jared Diamond if you haven't yet (where I got most of my info)!

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Theben
Co-Moderator of the Civ3 Forums

Also I think I solved the geographical distribution of diseases problem. First there is the random seed method. But the next version I post will include something similar to the previous but include geographic separation too. I'll be working on a new version of this and the Disaster model for the next few days. So expect it soon.

The benefits would obviously be the elimination of the disease but would also include a diplomatic improvement for the major contributors. A happiness improvement for a period of time should also be considered.

Not that i'm saying u have to use strains but how will u model the following situation (i will be using strains for the example so u can tell how to do it without):

Influenza Strain 1 in egypt
Influenza Strain 2 in greece

The egyptians are resitant to strain 1 but not strain 2 and vise versa.

They meet and combine to form strain 3, which is more powerful (this part has been explained well enough). Strain 3 then exists alongside 1 and two.

The Persains didn't start out with influenza (kinda unrealistic considering the geographic relationship but its only an example). Instead the egyptian strain was brought to them. Since the persians never had influenza they'd have a low resistance (again this part has been adequetly figured out). Now because the persians are slightly differnt from the egyptians aftern X turns they strain mutates into strain 4.

Quick Recap:

Strain 1: Greece
Strain 2: Egypt
Strain 3: Greece & Egypt (Hybrid)
Strain 4: Persia (Mutated Egypt Strain).

Now in order the resistance comparison of all the cultures, from most resistant to least:

Egypt: 2, 4, 1, 3
Greece: 1, 2, 3, 4
Perisa: 2, 3, 4, 1

Finnaly sveral years have gone by and after much mixing of blood and such. The earlier strain will almost always be weaker than earlier strains so strain 3, after it once again evolves to suit the egyptian population and becomes strain 5. If strain 5 goes to persia at the same tome the little that's left of strain 2 does, strain 5 will have a much better chance of floursishing than strain 2, which probably wouldn't at all.

Eventually strain 1 and 2 may become extinct (as well as the others) but the thing is although the diseases continue to increase in potency, the increase in potency only to that culture and a strain from another culture vastly differnt would usually have much more drastic effects, even if its the same disease.

If u can come up with a model that does that without strains i'm all for it.

Krenske:

I'm with you on all three points. If you get to the point where you've done all you think you can on diseases and natural disasters until the other models are further along, just let me know! I'm sure we can find additional productive work for you to do

Krenske:

Strains:
LGJ brings up a few points I hadn't even considered about them and also seems to understand what I wrote about them alot better than I do! So until we get more opinions on it other than just LGJ, Mark and myself I'll leave them in.

The reason is: croop diversity and diffusion is quite similar to the disease spreading being modelled in with this strain approach.
When Cultures of different geographic origins (here the geography and not etnography is important) achieve contact one of the first things to switch hands are desieses and croop knowledge (Potatoe :-).
The efficiency of this trade will depend on the ability to produce the crop in the homeland (if you have favourable climate you use the crop and produce more, if not you drop it)
Large geographically diversified regions with plenty of internal culture contacts achieved higher agricultural productivity indexes (Europe+Asia) than regions with either climateric barriers (Africa with several climateric regions from north to south) or geological and geographical limitations (North and South America have limited East-West dimensions which limit the crop development)

Now how to make this use of this in gameplay. Get something like the strain model into croop exchange; there would be great agricultural bonus in the first croop exchanges and they would decrease with further exchanges (this would be both realistic and fun to play because you would get civilizations like the Meso-Americans to develop despite somewhat limited crop diversity, encourage isolated cultures to establish contact with other groups, no hiding and achieving space tech for small island civs; this would also give a little bonus to the cultures that are allways banging heads with someone else because they are in the geographical center of global politics.

I´ll take a shot at modeling this idea.
will send it to you so that you may do lots of "constructive criticism" :-))

*Clearly most disease came from central Africa, Egypt, Arabia or India. The only exceptions are those few diseases from Europe (The first “official” cases of the disease I could find, most of which were well beyond 1A.D., but they most likely also came from Africa and just went by a different name earlier in history.), and Syphilis which some believe came from America but this is highly disputed. This basically suggests one disease pool containing all diseases in central Africa, and they simply spread from there.

So basically what I’m suggesting is rather than random distribution of disease we use a single large disease pool. Diseases would then simply spread along with migrations, armies, explorers and trade routes, as they seem to have in the past. This I believe would simulate real world history best.

As far as the single disease pool that could be okay for human diseases since that also is supported by one of the major theories of the evolution of man. There may be a few exceptions. The only thing i can think of that might contradict that idea would be that many of these diseases came from animals and adapted for human hosts. This is true with most viral diseases such as AIDS/HIV. Also some diseases in the american and asian jungles are local also. Many of these diseases only started to appear with the tearing up of these places. So what I say is that the diseases might not originate from 1 location, but should originate from tropical regions.

There is also genetically engineered diseases which don't fall under this category at all.