1 Epidemiology I: Principles 173:140Section 8: EPIDEMIC INVESTIGATION A. Definition of an Epidemic 1. An increase in the number of cases over past experience for a given population, time, and place 2. Description by Wenzel (Am J Infect Cont - Vol 16, Oct, 1988, p. 219) "The interaction of man and microbe - what we call disease - is a natural part of life; two species merely competing for survival within the same environment; and when the microbe wins in overwhelming numbers, human beings refer to that state of affairs as an epidemic, a word derived from the Greek upon the peoples." B. Essential Ingredients of an Epidemic 1. Introduction of, or an increased amount of, a pathogenic agent 2. An adequate number of exposed and susceptible persons 3. An effective means of transmission between the source of the pathogen and the susceptible hosts C. Common Circumstances for an Epidemic 1. When susceptibles are introduced into an endemic area where the pathogen exists (e.g., travelers) 2. When a new pathogen is introduced by traveling humans or animals from an endemic area into a susceptible human population (e.g., small pox introduced to American Indians from European settlers) 3. When contamination of food, water, or other vehicles takes place by an agent not normally present (e.g., cyanide added into Extra Strength Tylenol capsules) 4. When new and effective contacts are made between a preexisting infection of low endemicity and susceptible persons as a result of changes in social, behavioral, sexual, or cultural practices (e.g., measles outbreaks in U.S.) 5. When host susceptibility and response are modified by natural or drug-induced immunosuppression, nutrition, or other diseases (e.g., transplant patients) 6. Combinations of the previous circumstances.
2 D. Common Circumstances for Cessation of an Epidemic 1. Pathogen is eliminated or modified 2. Mode of transmission is interrupted or eliminated 3. Number of exposed and susceptible persons is markedly reduced or exhausted 4. Cofactor or some other important risk factor is modified or eliminated E. Types of Epidemics 1. Common source epidemic - involves exposure of susceptible persons to a common source of the pathogen (e.g., contaminated drinking water) 2. Point epidemic - involves disease occurring among susceptible persons at the same time to a common source of the pathogen (e.g., food poisoning) 3. Propagative, or progressive, epidemic - involves the transfer of the pathogen from one host to another 4. Mixed epidemic - involves both a single, common exposure to a pathogen and