How Evolution by Natural
in 2 Populations
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 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.
A. Human "ecosystem" is too incompatible - organisms cannot live
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
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 human's 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 (that's 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 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.
STEP ONE: INITIAL OUTBREAK.
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 organisms' 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 population's
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.
STEP TWO: THE OUTBREAK
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 are they spread through the population. For longterm survival,
the organisms must move to a new human population: they must get to
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 must 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...
STEP THREE: THE OUTBREAK RETURNS,
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, returning when enough new children
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 childhood resistance may not persist
into adulthood (by then the host will be immune, so it doesn't
matter), so adult exposure 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