This chapter considers what a
disease is, and why some microbes have evolved so as to make us sick, whereas
most other living things don’t make us sick. Later, the author goes on to
explain why many familiar diseases run in epidemics, such as AIDS. The chapter
will then conclude with insight into how animal origins of our infectious
diseases help explain the one-way exchange of germs between Europeans and
Native Americans.
The author begins the chapter with a story of
his physician friend who had to examine a small, timid, sick man with pneumonia
that was caused by some unidentified microbe.
In seeking to understand how the man became sick, the doctor asked the
man’s wife if her husband had any sexual experiences that could have caused the
infection. We find out that the man had sex with multiple sheep when he
returned to his farm back home The point of this story was to illustrate a very
important subject, which was human diseases of animal origins. Now let’s get into the evolution of all of
this. Basically, microbes evolve like
other species. Evolution selects those
individuals most effective at producing babies and at helping them spread to
suitable places to live. Microbes have evolved diverse ways of spreading from
one person to another, and fro animals to people. The germ that spreads better
leave more babies and ends up favored by natural selection. Many of human
symptoms of disease actually represent ways in which some microbe modifies our
bodies such that we become enlisted to spread microbes. So, from out POV genital sores and coughing
are symptoms of diseases, but from a microbe's POV, they are evolutionary
strategies to broadcast the germs, and this is why it is in the germs interest
to make us sick. Now, the author goes on
to examine how out bodies try to stay alive and healthy in response to microbe
caused infections. This includes, fever, mobilizing our immune system,
vaccination, and the slowest response being natural selection, which changes
out gene frequencies from generation to generation. For any diseases, some people tend to be more
resistant than others, so if an epidemic were to come, these people would be
the survivors. As a result, human populations repeatedly exposed to a
particular pathogen have come to consist of a higher proportion of individuals
with those genes for resistance. Examples of genetic defenses include
protecting northern Europeans against malaria. Because of these evolutionary
defenses, microbes have had to come up with tricks to let them spread between
potential victims, and many of these tricks are what we experience as symptoms of
disease. Later, the chapter goes on to characterize infectious diseases that
visit as epidemics rather than cases.
The four characteristics are: first, they spread quickly and efficiently
from an infected person to a nearby healthy person, with the result that the
whole population gets exposed; second is that if that they're acute illnesses
that one either dies from or completely recovers from; then there are the
fortunate ones who do recover and develop antibodies that leave us immune
against a recurrence of the disease for a long time; finally, these diseases
tend to be restricted to humans, meaning that the microbes causing them don’t
live in the soil. These four traits apply to what we think of as epidemic
diseases, such as smallpox or mumps. After explaining why these diseases run as
epidemics, the author goes on to explain why small populations only have
certain types of diseases rather than epidemics, while larger populations
have crowd diseases- diseases that need
human populations that are sufficient in number and densely packed. The build up of human population, needed for
crowd diseases, began with the rise of agriculture and rise of cities. This
leads to the question as why did agriculture launch to the evolution of crowd
infectious diseases? The answer: the rise of farming and cities was a bonanza for microbes because
of the build up of numerous people coming together, fecal matter being spread
as fertilizer which spread microbes, and forests clearings that provided ideal breeding
habitats for malaria- transmitting mosquitoes.
Shortly after, we learn about the four stages of evolution of a
specialized human disease from an animal precursor. These include: diseases
directly from animals (leptosperis from dogs); diseases that transfer from
human to human but die out (Fort Brag Fever) ; diseases that transfer form
human to human but have been established for very long (AIDS), and diseases
that have been long established which evolve effectively to work in their new
hosts (syphilis). To conclude the chapter, the author talks about the role of
diseases in conquest. He tells us how diseases played a significant role in
conquering the Aztecs and Incans when the Spaniards arrived to the Native Americans’
lands. The author explains that one of the main reasons many Native Americans
died was really due to the diseases that the Spaniards brought with them.
Diamond, Jared M. Guns, Germs, and Steel: The Fates of Human Societies. New York: W.W. Norton, 1999. Print.
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