272 CHAPTER 11 | The Emergence of Cities and States
and fresh air. Before humans lived in dark, crowded urban
centers, if an infected individual coughed and released the
TB bacteria into the air, sunlight would prevent the spread
of infection. Therefore, TB, like many other sicknesses,
can be called a disease of civilization.Social Stratification and Disease
Civilization affects disease in another powerful way. Social
stratification is as much a determinant of disease as any
bacterium, past and present. For example, Ashkenazi Jews
of eastern Europe were forced into urban ghettos over sev-
eral centuries, becoming especially vulnerable to the TB
thriving in crowded, dark, confined neighborhoods. As
we have seen with the genetic response to malaria (sickle
cell and other abnormal hemoglobins) and bacterial di-
arrheas (the cystic fibrosis gene), TB triggered a genetic
response in the form of the Tay-Sachs allele. Individuals
heterozygous for the Tay-Sachs allele were protected from
this disease.^19
Unfortunately, homozygotes for the Tay-Sachs al-
lele develop a lethal, degenerative condition that remains
common in Ashkenazi Jews. Without the selective pres-
sure of TB, the frequency of the Tay-Sachs allele would
never have increased. Similarly, without the strict socialmakes carriers resistant to cholera, typhoid, and other bac-
terial diarrheas.^17 Because of the mortality caused by these
diseases, selection favored spread of this allele among the
population. But, as with sickle-cell anemia, protection
comes at a price. That price is cystic fibrosis—a usually fa-
tal disease present in people who are homozygous for the
altered gene.
The rise of towns and cities brought with it other acute,
infectious diseases. In a small population, diseases such as
chicken pox, influenza, measles, mumps, pertussis, polio,
rubella, and smallpox will kill or immunize so high a pro-
portion of the population that the virus cannot continue to
propagate. Measles, for example, is likely to die out in any
human population with fewer than half a million people.^18
Hence, such diseases, when introduced into small com-
munities, spread immediately to the whole population and
then die out. For these diseases to continue, they require a
large population, such as is found in cities. Survivors pos-
sessed immunity to these deadly diseases.
The disease tuberculosis (TB) would not have become
widespread without the development of cities. The bacte-
ria that cause TB cannot survive in the presence of sunlight
East of Naples, Italy, dumping
of toxic waste and the sheer
volume of normal garbage
have become serious envi-
ronmental threats. Organized
crime syndicates provide il-
legal, less expensive ways to
dispose of toxic waste, which
has led to a contamination of
the environment and the foods
produced there. Dioxin, asbes-
tos, and other toxins may have
made their way into the food
supply, including into water
buffalo milk that is used to
make the gourmet food buffalo
mozzarella. In 2008, when the
crime syndicate–run landfill
reached capacity, garbage col-
lectors went on strike leaving
uncollected trash to pile up in
the city.
© AFP/Getty Images(^17) Ridley, M. (1999). Genome, the autobiography of a species in 23 chapters
(p. 142). New York: HarperCollins.
(^18) Diamond, J. (1997). Guns, germs, and steel (p. 203). New York: Norton. (^19) Ridley, p. 191.