Disease Using the Human Infrastructure
Dramatic changes in living conditions and population structure are usually associated with the changes caused by the Industrial Revolution. Massive migration to cities, and the development of urban centers which followed increased the likelihood of disease spread and evolution, and has also increased disease persistence. Diseases use cities (places where hundreds, thousands and millions of people are in very close quarters with each other) as a super highway; cities provide the perfect infrastructure for disease travel. As they travel and harm, diseases are in a constant race with their hosts towards evolutionary perfection. As their hosts evolve to kill the microbes, the microbes evolve to either keep the host alive for longer, or travel more quickly between hosts.
One way that disease has utilized the growth of cities in order to evolve
and thus travel better is its transformation from an exclusive disease for animals
to an exclusive disease for humans. For example, typhus was originally transmitted
between rats by fleas until typhus microbes realized that human body lice was
a much more efficient method of traveling, now that humans are no longer host
to lice typhus has changed to infect eastern North American flying squirrels
and then transferring to people who live in close proximity to the squirrels
(Diamond 209). Diseases are, in short, constantly changing in order to propagate
more efficiently and more quickly. Our intimacy with animals has provided a
quick and easy method of disease transformation and therefore better propagation.
Pathogens that were formerly secluded to animals evolve to the point where they
are directly transmitted between people. When these people are parts of large
communities (a contingency that diseases thrive on) epidemics result, especially
if the sanitation is as bad as in the first cities. In fact, up until the 20th
century, Europes urban population was not self sustaining, so many died
of crowd diseases that they had to be constantly replaced by rural immigrants
(Diamond 205). Many of our epidemics could not have existed without the cities
and the easy transportation they allow.
Diseases, like all organisms, are constantly evolving in order to pass on the
most genes. The best strategy for doing so is to replicate rapidly. If more
rapid replication of a microbe inside a person leads to greater passing on of
the genes that code for that rapid replication, then replication rate will increase
even if it causes the person to be severely ill or leads to an overall decrease
of the number of people it can effect, or even if it hastens the eventual extinction
of the microbe (Ewald). This is how epidemics develop. Epidemics spread quickly
and efficiently, they either kill the person or the person develops antibodies
- in short, no one is left to be infected after the disease has spread. So diseases
have developed very clever ways to move around the human infrastructure. They
modify our behavior (as with the rabies virus), hitchhike in insect saliva (as
with malaria), modify our anatomy (as with smallpox), or force us to deliver
the disease to the water supply (as with cholera)(Diamond). Humans, of course,
have counter evolved in order to survive, and therefore as long as humans live
in conditions in which disease can travel, we will be locked in an evolutionary
Diamond, Jared, Guns, Germs, and Steel" W.W. Norton & Co, 1997.
Ewald, Paul W. Evolution of Infectious Diseases. Oxford University Press, 1996.