How Evolution by Natural Selection
in 2 Populations Produces
Childhood Diseases.


A childhood disease is a disease that is (or was, prior to widespread vaccination) most common in children, and usually produces milder symptoms in children than adults.

These diseases evolved mostly in Europe and Asia during the Dark and Middle Ages and spread from there.  The two populations involved were those of the disease organisms and the human hosts.

If you've ever wondered why the colonial powers spread diseases to new lands but didn't bring new diseases back, this is why there were almost no virulent diseases to carry home.




A microbe (most likely a bacterium or virus), which we'll call the organism, uses another living thing, usually a wild or domestic animal, as a host. If the relationship has existed for a long time, the presence of the organism has minimal effects on the host.

Over time, the organism gets the chance to "jump" to humans. Possible results:

A.  Human "ecosystem" is too incompatible - organisms cannot live there.

B.  Organism and offspring can survive in a human (probably sickening and killing the host), but cannot pass offspring to new human hosts.

C.  Organism and offspring can survive in a human, and could move offspring to new human hosts, but its presence sickens and kill the human host too quickly and/or the passage to new hosts is not efficient enough.

D.  Organism and offspring can survive in a human, can move to new human hosts. Organism does not sicken host enough to prevent passage, or passage is accomplished before host is too sick to block it.  For example, cholera organisms quickly and purposely dehydrate the host, often killing them within a few days, but the extra-wet diarrhea that results increases the chance that offspring will survive long enough to reach more human drinking water, which is the route it uses to spread.

Because the organism is well-adapted to its current host, A is the most common result (this is why most diseases that do cross the species barrier move between closely-related species or between chemically-similar body systems, like omnivore digestive systems), and in those cases when it does successfully jump, its presence disturbs the humans' physiology, producing major disease symptoms - B and C. D is a very unlikely case, but there are organisms that seem to fit that category, such as the protozoan Giardia.  There are also some multi-compatible groups, such as influenza viruses, which can jump among people, pigs, and birds (thats why strains are often labeled "swine" or "bird" flu).

For the situation of the evolution of childhood diseases, C is the preliminary situation: an organism that exists in the environment, to which humans are repeatedly exposed but are largely incompatible with. The population of organisms is not uniform; due to mutations and gene transfer, there will be individuals that are more or less capable of surviving in some humans, individuals that disturb human physiology to a greater or lesser degree (make them more or less sick), and individuals that are better able to get offspring from one human to another.

The second required factor of this scenario:  human hosts must be packed together in large numbers in cities, and movement between cities must be frequent and not take very long. 



A human host picks up an organism that can survive in the human and spread to others. The organism may sicken the human, even greatly, but can't kill the host until its offspring have moved to another. This requires some critical time window to build up sufficient numbers of offspring to do this (we'll call this the transmission time window), and the level of sickness developed will greatly affect the organism's ability to spread. Having many potential hosts housed very close together really helps; organisms that invade humans without cities must be fairly gentle or very slow-developing right from the beginning.

The first wave of the outbreak is dramatic, probably with a high number of casualties. Humans that quickly die are probably less compatible with the organism; survivors are likely to have physiologies that can better tolerate the organism's presence - this is called resistance. Survivors' children will likely be more compatible with the organism than the original population as they will mostly inherit their parents' resistances.

As organisms produce offspring in humans and spread to new hosts, this population will have variant individuals due to mutation and gene transfer (and the transfer may come from organisms that successfully live in humans with minimal effects). Variants that are more compatible with human physiology are more likely to build up a population to the transmission time window and are less likely to severely sicken the host, so over time these milder variants become more common and the virulent strains become less common. There are complex factors here: a more-severe variant that can move to new hosts before the milder ones will still have an advantage, and this will be affected by external factors, such as the human populations size and density. Variants that increase the chance of passage also are favored, so organisms that can produce symptoms such as coughing, or skin lesions, from which offspring can easily move to new hosts are going to become more common. Many disease symptoms are not your body effectively fighting the disease, but are produced by the disease organisms to facilitate their spread.  For example, the common cold has evolved to be pretty benign, but the viruses have to make you cough or they won't spread very well.

In the starting city, the human population will shift toward resistant survivors and their children, and the unexposed. Modern medical knowledge has made it easier to increase that last fragment of the population, but in the Middle Ages there would have been fewer in that group. 



In the outbreak city, the survivors will develop antibodies to the organisms: they will become immune to reinfection. The organisms will have fewer and fewer potential human hosts as they spread through the population. For longterm survival, the organisms must move to a new human population: they must get to another city.

To get to another city, the organisms must be carried by human hosts. This will select the variant organisms that can best keep the human hosts "healthy" long enough to make the trip. Even though the traveling hosts are probably particularly resistant, the new outbreak almost has to start out as less virulent than the first outbreak. Because many humans in the new city will not have resistance even to this new variant, it is likely to still kill a significant number of hosts, and change the nature of this city's descendant human populations toward a resistant physiology (this is what happened when European explorers and colonists encountered new populations elsewhere, and when Asian traders came to Europe).

The basic process repeats, with the organism population becoming more mild (as long as its transmission capability is maximized) and the human populations becoming more resistant, then largely immune to reinfection. This requires many cities, relatively close together; the organisms must move to unexposed populations. If hosts carry organisms back to previously-exposed cities, too many humans will be immune to support a new outbreak. For a while... 



For a new outbreak in a previously-exposed city, a new, unexposed population must be present. If enough time has passed, that population will be children. They will be mostly children that inherited their parents' resistances (the ones who didn't are most likely to be killed in the new outbreak) but have no immunity (immunity can not be passed to children), since they haven't been exposed to the organisms (modern vaccination is a simulated exposure that gives a child immunity without giving them the disease).  Over time, children will be the vast majority of potential hosts as the disease moves from city to city, rising to outbreak level when enough new children are present.  A similar concept applies to what is called herd immunity if enough members of a modern community is vaccinated:  new victims can't easily find susceptible hosts to spread the organisms to.

Children are not physiologically identical to adults, so new variants in both organisms and hosts are favored: organisms that can live and spread among children, and humans that have greatest resistance as children. By this time, the organism will produce a disease mild enough that it doesn't need huge numbers of densely-packed hosts; it can spread to smaller population centers or into the countryside. What may also happen is that the physiological "quirks" that produce childhood resistance may not persist into adulthood (by then the child host will be immune, so it doesn't matter), so exposure for adults that never had the disease as children may produce much more extreme sickness.

At this point, a childhood disease has evolved by natural selection of genetic variants in both the disease organism population and the human population.




Copyright 2009, 2016, Michael McDarby.

Reproduction and/or dissemination without permission is prohibited.  Linking to this page is fine.


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