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heardie
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DISEASE MODEL
One of the most important aspects, I believe, is to have
a disease system, that is realistic enough for a great game,
yet simply enough for everyone to be able to understand it.
The system should not rule over the entire game, IMHO, as
there are more important things to concentrate on, but even
with this said….
Why have diseases:
The problem with earlier civilization games is that they all
fail to realize actual events, which cause actual problems.
In Civ and Civ2 both by Microprose, there aren't any plagues.
In real life, however, the Bubonic Plague swept through Europe
killing over 50 million people. Smallpox swept through the
Americans after being introduced by Cortez's men leaving only
10% of the
population alive. Even as recently as WWI over a quarter of
the world's people were infected by a pandemic of influenza.
Malaria, AIDS, and a whole plethora of others both new and
ancient have been a boon of civilizations throughout history.
This absolutely MUST be a factor in game design, especially
in that it provides a totally new and different enemy for
players to deal with. This would provide an ultimate solution
to a host of problems. First, ICS would become
suicide (by facilitating the rapid spread of infection). Also,
military oriented civs would be caught offguard with an inability
to deal with the new terrors.
Diseases affect different people in different ways. Each disease
needs to have the following characteristics:
Infectiousness: 1-10 with 1 being E-coli which doesn't spread
on it's own and 10 being Influenza which is airborne. This
is the rate at which the disease is spread and generally modifies
the quarantine variable.
Incubation: 1-10 with 10 being several years and 1 being just
a few days. 10 diseases would spread throughout the whole population
before any notice is given to the player and would take a
while before the effects take shape while 1 diseases can
wipe out whole squares before getting a chance to spread.
Mortality: 1-10 with 1 meaning only a few physically weak
people might die and 10 meaning a 100% mortality rate, no
survivors. (not necessarily everyone gets sick, of course)
Cureability: 1-10 with 1 being a simple parasite that can
be washed off, 2 is a parasite that requires some special
treatment, 3 is a bacteria that can be treated with herbs,
10 is a retro-virus for which there is no cure at present.
Certain advances can adjust this number- the discovery of
penicillin etc. With new strains, the higher number here indicates
resistance to earlier treatments.
Dependence of Environment(DOE): This tells, how dependent
it is of the conditions at the location it was born. Also
for each disease would be stored the climate type of the spawning
site. Diseases with dependence factor of 1could spread only
to the similar climate areas, while with 10 they could spread
anywhere.
Of course these numbers can change, after x number of turns
treating "disease c5" with penicillin, it's curability
number would be raised by 1 in a new strain and renamed "disease
c6" NOTE: Except for some, i.e. HIV, these are all based,
on when the disease first appeared, i.e. Smallpox is very
preventable now, but in the early years, it was much deadlier.
HIV:
Infectiousness = 3
Incubation = 8
Mortality = 10
Cureability = 10
DOE = 10
Influenza:
Infectiousness = 10
Incubation = 5
Mortality = 1-4(var)
Cureability = 9
DOE = 4
Smallpox:
Infectiousness = 7
Incubation = 5
Mortality = 9
Curability = 6
DOE = 8
Pneumonia:
Infectiousness = 0
Incubation = 5
Mortality = 8
Curability = 6
DOE = 4
These are only a few possibilities. Based on a civilizations
medical knowledge index the chance of detecting increases,
mortality rate decreases and the number you get for being
able to cure a disease increases. The perfect example of this
is Smallpox, which is these days 99.9% eradicated. In fact
the only known storage of Smallpox is being kept, in case
the disease ever surfaces again. This was meant to be destroyed
last year, which would have got rid of it forever, but they
decided to hold onto it for a year or two longer.
Each square will have a negative value by default, which
means that there are no diseases in that square. If it does
have a disease than it is a disease like AI, or B3.
TileA: Dis,-
TileB: Dis,-
TileC: Dis,A3
TileD: Dis,A3,B2,F1
TileE: Dis,-whereas in gameplay "Disease A" might
be named "Smallpox", and three is the particular
strain of that disease. Any undected diseases would have an
X% chance of spreading to an adjacent square depending on
infrastructure, that chance would be reduced by half if that
square is quarantined the same turn it is detected and reduced
by half again each turn after (after it's already too late,
of course =). Cities connected with active trade’s routes
would have a 99% chance of spreading the disease.
Aqueducts will reduce the chance of an outbreak occurring
on that square by 50%, but will facilitate a spread by doubling
the chance if an outbreak does occur.
Any infected units (unmarked during gameplay) automatically
spread the disease to everything it touches. Imagine the terror
in a player when he receives the message "You Legion
has been eradicated by the Plague" when that player realizes
that same legion has been through the heart of his empire.
Consequently, there needs to be a time lapse between infection
and detection.
In addition to this, there can be a section of your research,
where if you have a certain disease, then there can be an
option to research a cure/vaccine for the disease, i.e. what
is happening with AIDS and Cancer at the moment. Then after
a certain number of years, you will have discovered a cure.
Now this leaves you with a few opportunities. You could use
the disease
for biological warfare, knowing that if your enemies reciprocate
with the same disease you will have a vaccine/cure to it.
Alternatively, you could sell it to the rest of the world,
which would be a handy benefit to your wallet! The price the
rest of the world is willing to pay would depend on the mortality
of the disease. A cure/vaccine, for something like AIDS, would
sell for much
more than a cure for the common cold, although both would
be handy! The final
option, the ‘gentleman’ option would be to give
it away to the rest of the world, which should give you a
bonus to your reputation. Just on reputation I believe it
should be on a scale of 1-100, with 1 being you are the nicest
person, and 100 being you are a sadistic bastard. If you give
away the cure/vaccine than you will be a few points taken
away from the scale. In fact, I might start
a whole reputation thread later!
Civ A’s immunity to certain diseases would be different
to Civ B’s. Let’s set up a hypothetical:
Civ A = Romans
Civ B = Indians
The Indian Unit has a bad case of smallpox. He meets the
Roman unit, who exchange words, but then return to their own
civilizations. Let’s say the Roman goes back to Rome,
which has around 1 million people.
Smallpox is a deadly disease with the following characteristics:
Infectiousness = 7
Incubation = 5
Mortality = 9
Curability = 6
As the Infectiousness is 7, than 700,000 people of Rome will
get the disease. The amount of time it takes to get the disease
will start with a figure, and then decrease the next turn,
i.e.:
1st Turn back – 100,000
2nd Turn back – 300,000
3rd Turn back – 100,000
4th Turn back – 75,000
5th Turn back – 50,000
6th Turn back – 25,000 and etc.
Out of the 700,000 that contracted the disease than 40% of
them will be cured, (100%-60%). This leaves us with 700,000
– 280,000, or 420,000. Then out of that 90% of them will
die, or 378,000.
In a case like this, there is also the possibility of a unit
of one Roman city, not knowing he had smallpox, entering another
city, and setting off the disease in there.
There will be certain things that the player can do to curb
this. As the Incubation is 5, then a certain period of time
will pass, say for example two turns, before the player is
aware of the epidemic. Then he has the option of doing something
about it, i.e. quarantining all the un-effected people, which
can help to stem the flow of this disease, or researching
for a cure/vaccine,
or even buying one, if someone has discovered it. . This is
just an extreme case that is an example of an epidemic. In
some cases only a percentage of all this will happen. This
could be determined by a random generator, and/or affected
by sanitation levels in the city. It can also be affected
by the player’s immunity to certain diseases. If the
city had had smallpox before, than he would have say, for
example, a 50% immunity to it, so then the effect would be
lowered by 50% and by all the other modifiers.
Some Civ’s would be affected by disease different to
other Civ’s would be affected by the same disease. Also
at the beginning of the game, some Civ’s would have certain
disease that other Civ’s wouldn’t have, like the
tropical cities, would often get malaria, while one’s
located not near rivers wouldn’t.
Here's another thing, Influenza always mutates. So every
year even though there may be some immunities developed by
earlier strains, each new strain makes prior immunities useless.
To model this, diseases could be named "A1" or "A2"
indicating two different strains of the same disease. While
a prior infection would provide some help, maybe a 10% bonus,
it would be of little use.
In the modern years, there can be research centers built
in each city that would allow you to research for cures into
diseases, without actually using it as your technology that
you are pursing.
Another idea: Perhaps there can be funding. I don’t
know what we are planning on doing on corporations, but maybe
if you are discovering an cure for cancer, then a corporation
can put in funding for it, and then if it is discovered they
get some sort of recognition. Don’t know if this will
work under a corporation system.
Most of this is not my ideas. Some of it is, but there are
some credits to people that need to go out:
Guildmaster – he started the original thread and this,
and some of the ideas are his, or have evoled from his ideas.
amjayee – another forum poster, and one of the main programmers
for GGGS:ToC.
Toubabo_Koomi – who is working on the Clash, disaster
and disease models.
[This message has been edited by heardie (edited December 10, 2000).]
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Guildmaster
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Up your butt and around the corner
Jun 2000
time: 05:14
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It is not only reasonable to suggest that 1000 people can spread smallpox, it's very very likely. You know that 90% of the native american population in mexico, and entire civilizations in the islands were decimated because of only 1 of Cortez men. No more, no less. It took only one infected man to bring a disease 3000 miles across the ocean and kill millions and millions of people. This same fate befell the Aztecs, Incas, and a host of others.
Here's another thing, Influenza always mutates. So every year even though there may be some immunities developed by earlier strains, each new strain makes prior immunities useless. To model this, diseases could be named "A1" or "A2" indicating two different strains of the same disease. While a prior infection would provide some help, maybe a 10% bonus, it would be of little use.
If we have a toggle for every disease in every square, would that make gameplay tedious for the computer to keep track of it all? I would assume a disease factor is negative by default, and then during an infection it recieves a positive variable carrying the name of the disease. Something like:
TileA: Dis,-
TileB: Dis,-
TileC: Dis,A3
TileD: Dis,A3,B2,F1
TileE: Dis,-
Otherwise we would end up with...
TileA: A1,- A2,- A3,- B1,- B2,- F1,-
TileB: A1,- A2,- A3,- B1,- B2,- F1,-
TileC: A1,- A2,- A3,+ B1,- B2,- F1,-
etc.
Just a thought
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heardie
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edit: The pictures dont woork so well. imagination needed
I have been thinking about this and for simplicity this is what I think that we should do.
Diseases should be split into 2 types
A.
Diseases that will kill a certain percentage of your population until you discover a cure for it. This involves things such as the Common Cold, and could be determined by your tech level, your infrastructure, etc.
B.
Disease that are epidemics. These can be worked out using the modifiers mentioned in the original thread. That way the map can be like this:
1st Turn
___ ___ ___
| - | | A1| | - |
|___| |___| |___|
| - | | - | | - |
|___| |___| |___|
| - | | - | | - |
|___| |___| |___|
| B1| | - | | - |
|___| |___| |___|
| - | | - | | - |
|___| |___| |___|
| - | | - | | - |
|___| |___| |___|
This is the best ASCII representation of this. Imagine that the sqaures are labelled in the form
(n,m) where m is the row, and n is the colum, i.e. The top middle square is (1,2). In actual fact disease A1, and B1, might be the disease's Influenzea and HIV. Lets imagine that B1 is the disease Hiv.
Influenza has the following properties:
HIV:
Infectiousness = 3
Incubation = 8
Mortality = 10
Cureability = 10
DOE = 10
Lets go through these and how they effect it. As the infectiousness is 3, this means it will take 3 turns for the disease to spread. It also means that (3x10=) 30% of the effected square will get the disease. Thus in three turns the disease may look like this.
There are actually two possiblities here, A and B
A
4th Turn
___ ___ ___
| - | | A1| | - |
|___| |___| |___|
| - | | - | | - |
|___| |___| |___|
| - | | - | | - |
|___| |___| |___|
| B1| | B1| | - |
|___| |___| |___|
| - | | - | | - |
|___| |___| |___|
| - | | - | | - |
|___| |___| |___|
The disease expands by 1 sqaure in 1 direction
OR
B
4th Turn
___ ___ ___
| - | | A1| | - |
|___| |___| |___|
| - | | - | | - |
|___| |___| |___|
| B1| | B1| | - |
|___| |___| |___|
| B1| | B1| | - |
|___| |___| |___|
| B1| | B1| | - |
|___| |___| |___|
| - | | - | | - |
|___| |___| |___|
The disease expands by 1 sqaure in all directions.
What is better, A or B?
The next modifier is 'Incubation = 8'
This means that it will be a certain amount of turn's before the user finds out about the disease. This means that, for eg, if we used system B for Infectiousness, anf if an incubation of 8 meant 8 turns, then the map would look like this:
___ ___ ___
| - | | A1| | - |
|___| |___| |___|
| B1| | B1| | B1|
|___| |___| |___|
| B1| | B1| | B1|
|___| |___| |___|
| B1| | B1| | B1|
|___| |___| |___|
| B1| | B1| | B1|
|___| |___| |___|
| B1| | B1| | B1|
|___| |___| |___|
This would be the first the player sees of the disease, meaning any units that were on there could have already been effected 
Now out of this in each sqaure 30% of the people in that square have the disease. As the mortality is 10 (and 100% x 30% is still 30%) 30% of the people in each square will die. As the cureability of this disease is 10, than all the people who contract the disease will eventually die.
This is my vision of how the disease model should work.
[This message has been edited by heardie (edited December 10, 2000).]
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The Joker
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Copenhagen, Denmark
Aug 1999
time: 06:14
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The model seems pretty good. However, I can't really think too much about it right now, since I have other things on my mind. What I think is, that we should keep the disease thing fairly simple, since we will be doing it tile per tile.
Just one thing:
The two disease types shouldn't be seperated entirely. What should happen is, that when a type B epidemic has wiped through a hex for some turns the mortality and infected people would drop, and it would end up as a type A disease. But if some people that has not previously been in contact with the disease does so it should become a type B disease again while spreading through the new hexes.
There are only a few exceptions to this rule, the only important of these being malaria: diseases which does not spread from human to human, but rather directly from animal/(plant) to human. Other disease like this would be cowmadness (that's the danish word for it translated into the english - somehow it does not sound right, but I don't know the english word for it. That disease that is a big thing in the EU right now, and might be connected with the Kreutzfeldt Jacobs disease) and salmonella.
I am not sure how to handle these diseases. But they could propably be treated like the other diseases. Only malaria would have high dependance of enviroment, and the two others would have so low infectousness/mortality they would be important.
------------------
"Damn those nazicommunists."
- McBain
GGS Website
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Leland
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If every tile holds information whether it has diseases or not, wouldn't that be an enormous amount of information to store?
My suggestion: make diseases population based, and give certain elements a feature of being infected (units, settlements, food...). Some diseases would be dependant on the location, here a sort of invisible "disease unit" could be used. Opinions?
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cavebear
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of the Pleistocene
Oct 1999
time: 00:14
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I have a few concerns about the way that diseases are being modeled. It appears that Disease is a generic thing, and moves by geographical diffusion (though I hasten to say that the methods for that seem fine).
It is more that I am concerned with the initial appearance of diseases specific to individual civs, and the consequences of infected humans meeting new civs.
As far as I can tell, all human diseases originated from exposure to animals; some wild, but most domesticated. In addition, some diseases are limited to specific terrains.
First, I think there should be specific diseases used in the game. Second, I think it is important to connect the initial appearance of those specific diseases to the domestication of (or other exposure to) the appropriate animal origins. Third, some specific diseases should be limited to appropriate terrains.
For example: Smallpox, Tuberculosis, and Measles came to us from cattle; Influenza from pigs; Malaria from birds; HIV from monkeys or chimps. For those diseases originating from domesticated animals (Smallpox Measles, etc), the initial introduction of them should be related to the domestication of those animals. For diseases which are geographically limited (Malaria, Sleeping sickness, Yellow Fever), then the effect and spread of it should be limited to appropriate terrains.
Whether limited by terrain or not, a specific disease should both diffuse gradually (and 2 rates of diffusion are fine) and by infected human movement. In other words, Smallpox should spread within a culture, but it should also be able to be carried to other areas by traders, armies, and explorers. It is natural that Smallpox would slowly spread among adjacent squares (because there are some resident humans throughout all the tiles), but it should also be able to be carried from Europe to the Americas by humans. Equally, Malaria should be limited to jungle terrains, though humans could spread it within jungle areas.
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alms66
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Louisiana
Oct 1999
time: 23:14
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Hi all,
I haven't posted much in GGS due to lack of time, but fortunately (at least for me ) I am nearly finished with school, and therfore have much more free time. So I would like to add to the above discussion, in response to Cavebear's post:
1. No disease is limited to a specific terrain, maybe by climate, but that is a different story if you add global warming to the picture, where warming may change terrain.
2. HIV came to us from Monkeys, if you accept that theory, there are several more out there, #1 being that is was a manufactured disease, though I personally doubt that.
3. Malaria limited to jungle terrains is a myth, at best, it is simply not true, in the 1990's there have been several outbreaks of Malaria in the U.S. (Texas mostly), but still, not jungle, and definately not jungle-like.
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S. Kroeze
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the Hague, the Netherlands, Old Europe
Dec 1999
time: 05:14
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quote:
Originally posted by amjayee on 01-02-2001 01:14 PM
I agree with cavebear mostly, this is how the system should work. But I personally doubt that all diseases are of animal origin. I guess we humans must have some diseases of our own, too? Or must have had? But I can see why most of them would come from animals; there are millions of animal species, humans are only one of them.
Perhaps someone who knows the mechanisms of the real diseases, and make a system of disease spawning and spreading. I'm willing to accept anything that works and is somewhat realistic.
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'In regard to causes of death, Weidenreich observed that most of the fossil remains of prehistoric man clearly indicate a violent death. For Paleolithic man of historic time, Krzywicki arrived at a similar conclusion, observing that the most frequent causes of death were infanticide, war, and headhunting.(!) Acsádi and Nemeskéri point out that, in general, death was closely related "to accidents, violence, and diseases resulting from factors of the natural environment or their changes". The low density of population was in a way a protection against epidemics. It is indeed difficult to see how, with spare populations organized into small bands wandering over limited territories, contagious diseases could have had the importance that they have assumed under other demographic conditions. But it is not diffcult to believe that starvation and diseases connected with nutritional deficiences must have taken on the whole a heavy toll of human life in Paleolithic and Mesolithic societies, especially among infants.'
(source: C.M.Cipolla:'The Economic History of World Population',1962)
'There were four main influences which led to the predominance of infectious diseases as causes of sickness and death:
- the existence, probably for the first time, of populations large enough to enable some human infections to become established and others to be amplified;
- defective hygiene and crowding, which further increased exposure to communicable diseases;
- insufficient food which lowered resistance to infection;
- and close contact with domesticated and other animals which were the probable source of many micro-organisms.
As already noted, early man was exposed to infectious diseases, particularly to the zoonoses, infections of other animals transmitted to man. But living in small groups of no more than a few hundred persons he must have been free of most human infections, and Burnet considered that under such conditions the infectious diseases which we now know did not exist.
The great change in community size is believed to have begun about 6,000 years ago; before that time human settlements consisted of villages with less than 300 persons, much too small to maintain the human infections. It was only after the introduction of improved farming techniques, particularly irrigated agriculture, that a few cities had 100,000 persons, and only in the last few centuries after industrialization that they had half a million. Remarkably, we owe the origin of most serious infectious diseases to the conditions which led to our cultural heritage, the city states made possible by the planting of crops in the flood plains of Mesopotamia, Egypt and the Indus Valley.
The conditions which resulted from agriculture had a profound effect on the frequency of exposure to infectious disease. The most important influence was the proximity of large numbers of people, which facilitated the spread of airborne and other infections. But hygienic conditions were also significant, particularly those determined by methods of handling food and water and disposing of excreta and waste. Cholera, as we have noted, appeared when villages and village water supplies were established, and malaria became serious when 'the size of human populations and the opportunities for breeding of vectors increased with advances in agricultural practices.' Tuberculosis, possibly an ancient disease, could almost be described as a disease of cities, for it became a common cause of death under the conditions prevailing in large towns. The spread of intestinal infections - typhoid, dysentery, tuberculosis, salmonella and the like - resulted from contamination of food and water. So the hygienic conditions which followed the introduction of agriculture made it possible for new diseases to appear, and for some diseases already present to become more serious.'
(source: T. McKeown: 'The Origins of Human Disease',1988)
'So-called epidemic diseases, though, produce no cases for a long time, then a whole wave of cases, then no more cases again for a while.
Among such epidemic diseases, influenza is one personally familiar to most Americans, certain years being particularly bad years for us (but great years for the influenza virus). Cholera epidemics come at longer intervals, the 1991 Peruvian epidemic being the first one to reach the New World during the 20th century. Although today's influenza and cholera epidemics make front-page stories, epidemics used to be far more terrifying before the rise of modern medicine. The greatest single epidemic in human history was the one of influenza that killed 21 million people at the end of the First World War.'
(personal note: I am sure that undernourishment of the European population contributed a lot to the devestating effects of this epidemic)
'The Black Death (bubonic plague) killed one-quarter of Europe's population between 1346 and 1352, with death tolls ranging up to 70 percent in some cities. When the Canadian Pacific Railroad was being built through Saskatchewan in the early 1880s, that province's Native Americans, who had previously had little exposure to whites and their germs, died of tuberculosis at the incredible rate of 9 percent per year.
The infectious diseases that visit us as epidemics, rather than as a steady trickle of cases, share several charcteristics.
- First, they spread quickly and efficiently from an infected person to nearby healthy people, with the result that the whole population gets exposed within a short time.
- Second, they're "acute" illnesses: within a short time, you either die or recover completely.
- Third, the fortunate ones of us who do recover develop antibodies that leave us immune against a recurrence of the disease for a long time, possibly for the rest of our life.
- Finally, these diseases tend to be restricted to humans; the microbes causing them tend not to live in the soil or in other animals.
All four of these traits apply to what Americans think of as the familiar acute epidemic diseases of childhood, including measles, rubella, mumps, pertussis, and smallpox.
The reason why the combination of those four traits tends to make a disease run in epidemics is easy to understand. In simplified form, here's what happens. The rapid spread of microbes, and the rapid course of symptons, mean that everybody in a local human population is quickly infected and soon thereafter is either dead or else recovered and immune. No one is left alive who could still be infected. But since the microbe can't survive except in the bodies of living people, the disease dies out, until a new crop of babies reaches the susceptible age - and until an infectious person arrives from the outside to start a new epidimic.
A classic illustration of how such diseases occur as epidemics is the history of measles on the isolated Atlantic islands called the Faeroes. A severe epidemic of measles reached the Faeroes in 1781 and then died out, leaving the islands measles free until an infected carpenter arrived on a ship from Denmark in 1846. Within three months, almost the whole Faeroes population (7,782 people) had gotten measles and then either died or recovered, leaving the measles virus to disappear once again until the next epidemic. Studies show that measles is likely to die out in any human population numbering fewer than half a million people. Only in larger populations can the disease shift from one local area to another, thereby persisting until enough babies have been born in the originally infected area that measles can return there.
What's true for measles in the Faeroes is true of our other familiar acute infectious diseases throughout the world. To sustain themselves, they need a human population that is sufficiently numerous, and sufficiently densely packed, that a numerous new crop of susceptible childred is available for infection by the time the disease would otherwise be waning. Hence measles and similar diseases are also known as crowd diseases.
Obviously, crowd diseases could not sustain themselves in small hands of hunter-gatherers and slash-and-burn farmers. As tragic modern experience with Amazonian Indians and Pacific Islanders confirms, almost an entire triblet may be wiped out by an epidemic brought by an outside visitor -because no one in the tribelet had any antibodies against the microbe. For example, in the winter of 1902 a dysentery epidemic brought by a sailor on the whaling ship Active killed 51 out of the 56 Sadlermiut Eskimos, a very isolated band of people living on Southampton Island in the Canadian Arctic. In addition, measles and some of our other "childhood" diseases are more likely to kill infected adults than children, and all adults in the tribelet are susceptible. (In contrast, modern Americans rarely contract measles as adults, because most of them get either measles or the vaccine against it as children.) Having killed most of the tribelet, the epidemic then disappears. The small population size of tribelets explains not only why they can't sustain epidemics introduced from the outside, but also why they never could evolve epidemic diseases of their own to give back to visitors.
That's not to say, though, that small human populations are free from all infectious diseases. They do have infections, but only of certain types. Some are caused by microbes capable of maintaining thmselves in animals or in the soil, with the result that the disease doesn't die out but remains constantly available to infect people. For example, the yellow fever virus is carried by African wild monkeys, whence it can always infect rural human populations of Africa, whence it was carried by the transatlantic slave trade to infect New World monkeys and people.
Still other infections of small human populations are chronic diseases such as leprosy and yaws. Since the diseases may take a very long time to kill its victim, the victim remains alive as a reservoir of microbes to infect other members of the tribelet. For instance, the Karimui Basim of the New Guinea highlands, where I worked in the 1960s, was occupied by an isolated population of a few thousand people, suffering from the world's highest incidence of leprosy- about 40 percent! Finally, small human populations are also susceptible to nonfatal infections against which we don't develop immunity, with the result that the same person can become reinfected after recovering. That happens with hookworm and many other parasites.
All these types of diseases, characteristic of small isolated populations, must be the oldest diseases of humanity. They were the ones we could evolve and sustain through the early millions of years of our evolutionary history, when the total human population was tiny and fragmented. These diseases are also shared with, or similar to the diseases of, our closest wild relatives, the African great apes.(!) In contrast, the crowd diseases, which we discussed earlier, could have arisen only with the buildup of large, dense human populations. That buildup began with the rise of agriculture starting about 10,000 years ago, and then accelerated with the rise of cities starting several thousand years ago. In fact, the first attested dates for many familiar infectious diseases are surprisingly recent: around 1600BC for smallpox (as deduced from pockmarks on an Egyptian mummy), 400BC for mumps, 200BC for leprosy, AD 1840 for epidemic polio, and1959 for AIDS.
While over a dozen major infectious diseases of Old World origins became established in the New World, perhaps not a single major killer reached Europe from the Americas. The sole possible exception is syphilis, whose area of origin remains controversial. The one-sidedness of that exchange of germs becomes even more striking when we recall that large, dense human populations are a prerequisite for the evolution of our crowd infectious diseases. If recent reappraisals of the pre-Columbian New World populations are correct, it was not far below the contemporary population of Eurasia. Some New World cities like Tenochtitlán were among the world's most populous cities at the time. Why didn't Tenochtitlán have awful germs waiting for the Spaniards?
One possible contributing factor is that the rise of dense human populations began somewhat later in the New World than in the Old World. Another is that the three most densely populated American centres -the Andes, Mesoamerica, and the Mississippi Valley- never became connected by regular fast trade into one huge breeding ground for microbes, in the way that Europe, North Africa, India, and China became linked in Roman times. Those factors still don't explain, though, why the New World apparently ended up with no lethal crowd epidemics at all. (Tubercolosis DNA has been reported from the mummy of a Peruvian Indian who died 1,000 years ago, but the identification procedure used did not distinguish human tuberculosis from a closely related pathogen (Mycobacterium bovis) that is widespread in wild animals.)
Instead, what must be the main reason for the failure of lethal crowd epidemics to arise in the Americas becomes clear when we pause to ask a simple question. From what microbes could they conceivably have evolved? We've seen that Eurasian crowd diseases evolved out of diseases of Eurasian herd animals that became domesticated. Whereas many such animals existed in Eurasia, only five animals of any sort became domesticated in the Americas: the turkey in Mexico and the U.S.Southwest, the llama/alpaca and the guinea pig in the Andes, the Muscovy duch in tropical South America, and the dog throughout the Americas.
In turn, we also saw that this extreme paucity of domestic animals in the New World reflects the paucity of wild starting material. About 80 percent of the big wild mammals of the Americas became extinct at the end of the last Ice Age, around 13,000 years ago. The few domesticates that remained to Native Americans were not likely sources of crowd diseases, compared with cows and pigs. Muscovy ducks and turkeys don't live in enormous flocks, and they're not cuddly species (like young lambs) with which we have much physical contact. Guinea pigs may have contributed a trypanosome infection like Chaga's disease or leishmaniasis to our catalog of woes, but that's uncertain. Initially, most surprising is the absence of any human disease derived from llamas (or alpacas), which it's tempting to consider the Andean equivalent of Eurasian livestock. However, llamas had four strikes against them as a source of human pathogens:
- they were kept in smaller herds than were sheep and goats and pigs;
- their total numbers were never remotely as large as those of the Eurasian populations of domestic livestock, since llamas never spread beyond the Andes;
- people don't drink (and get infected) by llama milk;
- and llamas aren't kept indoors, in close associations with people.
In contrast, human mothers in the New Guinea highlands often nurse piglets, and pigs as well as cows are frequently kept inside the huts of peasant farmers.'
(source: J.Diamond;'Guns, Germs and Steel',1998)
I hope this more or less answers Amjayee's question.
Another point: Time and again I have argued for the importance of adequate food supply and its relation to diseases. I can recommend reading several of my posts on this issue in the 'Population Model v. 0.1'
As a result Guildmaster came with the following intelligent idea:
quote:
What we could do is add a "threshold" to the health aspect of the population model. So if a sample of the population has a health of 6, that would mean that the disease has to have an infection rate of at least 6 to spread to that sample. 5 or less and it wouldn't spread there. Maybe a few insignificant cases to weed out the weak and unfit, 1% at most maybe depending on how close to the threshold the disease actually comes. 1% infection for 1 less, half for each point lesser. So a health of 6 vs. infectiosness of 3 means .25% (1% for 5, .5% for 4, .25% for 3, .125% for 2, etc). infected, anything 1% or less isn't enough to spread the disease.
Of course if the infection rate is equal to the health level, you get normal infection occur. If it's greater than the health level then you get it double up to a maximum of 99% of the population infected.
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I hope some of my information will be used!
Sincere regards,
S. Kroeze
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amjayee
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Jyväskylä, Finland
Oct 1999
time: 07:14
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S.Kroeze: Good information! Thanks for sending it, I'm sure it's of use. Yes, I remember well the excerpt from GGS, I think it especially had the point.
cavebear: Hmm... Is it really so, that the animals we have got some disease from, don't suffer from the microbe it has given to us? Let's take cow pox for example, that changed into small pox in our bodies. Is cow pox completely harmless for cows? I don't know this, but if it is true, it's quite interesting. Then another issue; have our animals got diseases from us? I mean, they have been in close contact with us, just as we with them? Wouldn't it be funny, if cows could get small pox, which is adapted to humans, and it would then re-adapt to cows, causing a completely new cattle disease. Just a vision, though, since small pox is no more.
I think what you said is not entirely true - those prion diseases that are so infamous currently, especially the one that has changed to CJD in humans (Creutzfeld-Jacob disease) certainly cause harm to cow brain too, as far as I have heard.
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19-06-2000 15:21
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