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Chapter outline
Chapter 2
Concepts of Infectious Disease and a History of Epidemics
This chapter begins with a short introduction to epidemiology that is complementary to
the more detailed discussion of the role of epidemiology in the AIDS epidemic presented
in Chapter 6. The reader is introduced to the term epidemic and the germ theory of
infectious disease. The factors that affect the spread of epidemics are discussed in the
next section; these factors include the number of susceptible individuals, the number of
infected individuals, and the transmission rate of the infectious disease. The reader learns
that the transmission rate of a disease depends on two components: inherent transmission
efficiency of the infectious agent and the number of encounters between infected and
uninfected individuals. The concepts of endemic disease, contagious disease, and acute
versus chronic infections are also discussed and these concepts are represented
diagrammatically in three simple figures. The term pandemic, used to describe an
epidemic occurring concurrently on more than one continent, is also introduced. The
chapter continues with a comprehensive review of the history of epidemics beginning
with the epidemics that occurred during the Greek and Roman civilizations, the black
death in Europe in 1346, the smallpox epidemic that decimated the Aztec population in
Mexico in 1518, and ending with epidemics in modern times including the influenza
pandemic of 1918 and the polio epidemic in the USA in the 1950s. The impact of the
AIDS pandemic is brought into perspective by comparing the number of AIDS-related
deaths that have already occurred with the deaths that resulted from some of these earlier
outbreaks of disease.
Within this section is a more detailed discussion of the modern concepts of infectious
disease, including Koch’s postulates, which are a set of experimental criteria developed
in the 1890s for defining the causal agent of an infectious disease. These criteria were
developed before viruses were discovered and it is pointed out that the four postulates are
too stringent to be directly applied to viruses that are obligate intracellular parasites.
Finally, the chapter concludes with a section discussing the parallels between AIDS and
syphilis, a sexually transmitted disease that was rampant at the turn of the 20th century. It
is striking that for many years, public health measures to control syphilis were not
implemented in the USA because of the stigma associated with the disease. It was not
until the 1930s that the then surgeon general of the United States, Thomas Parren,
effected changes in policy that brought the syphilis epidemic under control. It is pointed
out that preaching abstinence to control sexually transmitted diseases is not an effective
policy without other education and prevention strategies including the use of condoms for
those who either cannot or will not practice abstinence.