Supplemental Lecture (98/05/09 update) by Stephen T. Abedon (abedon.1@osu.edu)

  1. Chapter title: Epidemiology
    1. A list of vocabulary words is found toward the end of this document
    2. Epidemiology is a powerful means by which science answers important questions, concerning causes of disease , from population information (such as location and rate of increase in disease incidence). Thus, for example, the "acquired" in acquired immunodeficiency syndrome (AIDS ) was discovered by epidemiological means.
    3. While epidemiology can be very effective in ascertaining large effects, the significance of epidemiological conclusions when effects are very small is debatable. Consequently, we are bombarded with epidemiological data all the time, much of it contradictory. For example, does or does not living near high voltage electrical cables lead to a higher risk of cancer ? We simply do not know, probably because the relationship is small, subtle, or does not exist.
    4. Epidemiological data is often presented in a misleading manner. For instance, if exposure to chemical A increases 2-fold your likelihood of acquiring cancer B, does it really make any difference to you if your odds of acquiring cancer B in the absence of exposure to chemical A were 1 in 10 million? Not really. It would represent a jump from 250 cases per year in this country without exposure, to 500 with (again, assuming the effect is real). On the other hand, if the incidence is already 1 in 1000 and there is a doubling, that's an increase from 250,000 per year to 500,000 in this country. Clearly the latter result is the more relevant to your and the public's health.
    5. Such are the things one must keep in mind when interpreting the often irrelevant though sometimes inescapably relevant fruits of epidemiology (particularly, if you want to die prematurely, smoke cigarettes, drink to excess, and don't buckle your seat belt). These problems don't directly detract from the power of epidemiology, but do detract from the public's perception of the power of epidemiology. However, when serious effects loom large and we simply don't understand what is going on, it is very often the epidemiologists who play crucial roles in finding out what the problem is. Consequently, in this lecture we will delve, albeit quite shallowly, into the science of epidemiology.
  2. Epidemiology
    1. Epidemiology is the study of the
      1. occurrence
      2. transmission
      3. avoidance
      4. cure
    2. of disease .
    3. Occurrence of disease:
      1. Epidemiology is the science of understanding disease occurrence.
      2. Occurrence is in terms of distribution, both geographical and demographical, i.e., incidence and prevalence.
      3. In other words, how often, why, and when does disease occur?
    4. Transmission of disease:
      1. Epidemiologists additionally study how infectious diseases are transmitted within populations (e.g., where is the disease coming from? Which is the index case?).
      2. By looking at who is sick it is often possible to get a handle on how a disease is transmitted.
    5. Disease avoidance:
      1. Epidimiology also deals with determining factors that contribute to disease avoidance.
      2. Does a specific treatment work? Why does individual (or population) A get sick but not individual or population B?
    6. Epidimiology also deals with determining factors that contribute to disease cure (i.e., What works and what doesn't?).
  3. Incidence of disease
    1. Rate of contraction:
      1. The incidence of a disease is a number equal to the fraction of the population that contracts a given disease during a given period of time.
      2. In other words, the incidence of a disease is the rate at which new cases of a disease occur within a population.
  4. Index case
    1. An index case is the first case of a disease to be identified, either ever or at the start of an epidemic or a seasonal (or periodic) outbreak.
  5. Prevalence of disease
    1. The prevalence of a disease is the fraction of the population that currently (or at some other particular time) has a given disease .
    2. Not equivalent:
      1. Note that prevalence of disease and incidence of disease are neither identical nor equivalent measures.
      2. Instead, they refer to the rate at which people contract a disease (incidence) vs. the total number actually sick at any given time (prevalence).
  6. Sporadic disease
    1. A sporadic disease is one that occurs only occasionally in a population (i.e., prevalence is zero).
  7. Endemic disease
    1. An endemic disease is one that is always present in a population (i.e., never zero prevalence).
  8. Epidemic disease
    1. An epidemic disease is a disease that many people acquire over a short period (i.e., increasing incidence).
  9. Common source outbreak
    1. Some diseases arise from a single definable source, such as a common water supply.
    2. The basic idea is that common source outbreaks are not propagated from individual-to-individual (e.g., person-to-person). Instead, sick individuals typically are propagation dead ends.
    3. Yet the disease continues to be endemic and perhaps epidemic as a consequence of contact with some typically geographically well-defined disease reservoir.
  10. Propagated epidemic
    1. Contrasting with common source outbreaks are propagated epidemics, which are diseases spread not from some common, geographically well defined disease reservoir, but instead by individual-to-individual (e.g., person-to-person) contact.
  11. Pandemic disease
    1. A pandemic disease is a world-wide epidemic disease (i.e., high world-wide incidence).
  12. Emerging infectious disease
    1. An emerging infectious disease is a disease that is new to a host species or changing in some manner within a host species .
  13. Vocabulary
    1. Common source outbreak
    2. Emerging infectious disease
    3. Endemic disease
    4. Epidemic disease
    5. Epidemiology
    6. Incidence of disease
    7. Index case
    8. Pandemic disease
    9. Prevalence of disease
    10. Propagated epidemic
    11. Sporadic disease
  14. Practice questions
    1. A nosocomial infection whose prevalence has remained low and more or less constant in New York City hospital A, but is rising in New York City hospital B is an example of ____________ in hospital A. (circle only one correct answer) [PEEK]
      1. a sporadic disease
      2. an epidemic disease
      3. a pandemic disease
      4. an endemic disease
      5. all of the above
      6. none of the above
    2. Contrast incidence and prevalence of disease.[PEEK]
    3. Contrast sporadic with endemic disease (don't use the words sporadic or endemic, or their derivatives, to define these terms except in indicating which term you are talking about).[PEEK]
    4. Botulism occurs rarely and is not a communicable disease when it does occur. Assuming that the incidence of botulism remains unchanged, one would classify botulism as (circle only one correct answer) [PEEK]
      1. a sporadic disease
      2. an epidemic disease
      3. a pandemic disease
      4. an endemic disease
      5. all of the above
      6. none of the above
    5. A disease whose prevalence never reaches zero within a population is called a(n)__________ disease. [PEEK]
    6. Define endemic disease in terms of disease prevalence. [PEEK]
    7. An epidemic of gastrointestinal illness is traced to a single, out of compliance, restaurant kitchen. This is an example of an outbreak that may be traced to a single source. Another, general term for such an outbreak is __________. [PEEK]
    8. What is the typical prevalence of a sporadic disease? [PEEK]
    9. Contrast pandemic and epidemic. [PEEK]
  15. Practice question answers
    1. iv, an endemic disease
    2. Incidence is the rate of acquisition of a disease within in a population where prevalence is the fraction of the population which are afflicted. Thus, a disease could have a high prevalence but a zero incidence if there are plenty of old cases but no new ones.
    3. Both are diseases which are found in populations, endemic is always found in a given population while sporadic is found only from time to time.
    4. i, a sporadic disease
    5. endemic.
    6. An endemic disease is one which is always present in a population and therefore which has a prevelance which never drops to zero.
    7. common source outbreak
    8. zero
    9. A pandemic is an epidemic which occurs world-wide.
  16. References
    1. Black, J.G. (1996). Microbiology. Principles and Applications. Third Edition. Prentice Hall. Upper Saddle River, New Jersey. pp. 416-422, 433-435.
    2. Tortora, G.J., Funke, B.R., Case, C.L. (1995). Microbiology. An Introduction. Fifth Edition. The Benjamin/Cummings Publishing, Co., Inc., Redwood City, CA, pp. 382-389.