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The development of cities, and the concentration of people living in them, permit the rapid spread
of infectious disease-causing agents.
Social factors may also contribute to disease epidemics.
Highly concentrated urban populations promote the
rapid spread of infectious organisms among large num-
bers of people (}ÕÀiÊ{°È). Global travel also has the
potential to contribute to the rapid spread of disease as in-
fected individuals move easily from one place to another.
Consider malaria, a disease that mosquitoes transmit
to humans. Each year, between 200 million and 500 mil-
lion people worldwide contract malaria, resulting in
more than 1 million deaths. About 60 different species of
the Anopheles mosquito transmit the parasites that cause
malaria. Each mosquito species thrives in its own unique
combination of environmental conditions (such as el-
evation, amount of precipitation, temperature, relative
humidity, and availability of surface water).
Many other diseases are carried by animals, including
West Nile virus (mosquitoes), Lyme disease (deer ticks),
and the Bubonic plague (rats and fleas). Efforts to control
these diseases can have additional environmental impact.
For example, since mosquitoes reproduce in stagnant wa-
ter, they can be controlled by pesticides like DDT or by
filling in or altering swamps and ponds. These activities in
turn can have effects on other organisms. Also, the animals
that carry disease have proved to be highly adaptive—both
mosquitoes and malaria, for example, are now resistant to
many pesticides and medicines. Another recent concern
is pandemic influenza (flu). A pandemic disease reaches
nearly every part of the world and has the potential to infect
almost every person. Avian influenza is a strain of influenza
virus that is common in birds. It tends to be difficult for hu-
mans to contract because it is usually transferred from bird
to human but not from human to human. It is extremely
potent once contracted and has a high fatality rate.
In late spring 2009 a strain of the swine flu appeared
in Mexico, and by early summer was pandemic, killing
thousands of people worldwide. Flu and other diseases
generally spread more easily between related species:
Humans are more closely related to pigs than to birds.
(AIDS is thought to have originated from human contact
with diseased monkeys.)
The international response to the 2009 swine flu out-
break continues to be controversial. Some argue that,
since far fewer deaths occurred than predicted under
worst-case assumptions, we overreacted. Others think
that we should have done more, since we did not know
that the strain would be relatively mild.