Concepts of Infectious Disease and a History of Epidemics

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.