Principles of Epidemiology



History of Epidemiology

Epidemiology is the study of the patterns, causes, and effects of health and disease conditions in defined populations.

Learning Objectives

Describe the key events in the development of the field of epidemiology

Key Takeaways

Key Points

  • The Greek physician Hippocrates is known as the father of medicine, and was the first epidemiologist.
  • The distinction between ” epidemic ” and “endemic” was first drawn by Hippocrates, to distinguish between diseases that are “visited upon” a population (epidemic) from those that “reside within” a population (endemic).
  • In the early 20th century, mathematical methods were introduced into epidemiology adding statistical support to the field (i.e. the suspicion that tobacco smoking was linked to lung cancer was backed by statistics).

Key Terms

  • endemic: (Especially of diseases. ) Prevalent in a particular area or region.
  • epidemic: A widespread disease that affects many individuals in a population.
  • epidemiology: The branch of a science dealing with the spread and control of diseases, computer viruses, concepts, etc., throughout populations or systems.

Epidemiology is the study of the patterns, causes, and effects of health and disease conditions in defined populations. It is the cornerstone of public health, and informs policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive medicine. Epidemiologists help with study design, collection and statistical analysis of data, and interpretation and dissemination of results. Epidemiology has helped develop methodology used in clinical research, public health studies and, to a lesser extent, basic research in the biological sciences.

The Greek physician Hippocrates is known as the father of medicine, and was the first epidemiologist. Hippocrates sought a logic to sickness. He is the first person known to have examined the relationships between the occurrence of disease and environmental influences. Hippocrates believed sickness of the human body to be caused by an imbalance of the four Humors (air, fire, water and earth “atoms”). The cure to the sickness was to remove or add the humor in question to balance the body. This belief led to the application of bloodletting and dieting in medicine.

The distinction between “epidemic” and “endemic” was first drawn by Hippocrates, to distinguish between diseases that are “visited upon” a population (epidemic) from those that “reside within” a population (endemic). The term “epidemiology” appears to have first been used to describe the study of epidemics in 1802 by the Spanish physician Joaquín de Villalba in Epidemiología Española. Epidemiologists also study the interaction of diseases in a population, a condition known as a syndemic.

One of the earliest theories on the origin of disease was that it was primarily the fault of human luxury. This was expressed by philosophers such as Plato and Rousseau, and social critics like Jonathan Swift. In the middle of the 16th century, a doctor from Verona named Girolamo Fracastoro was the first to propose a theory that these very small, unseeable, particles that cause disease were alive. They were considered to be able to spread by air, multiply by themselves and to be destroyable by fire. In 1543 he wrote a book De contagione et contagiosis morbis, in which he was the first to promote personal and environmental hygiene to prevent disease. The development of a sufficiently powerful microscope by Anton van Leeuwenhoek in 1675 provided visual evidence of living particles consistent with a germ theory of disease.

Dr. John Snow is famous for his investigations into the causes of the 19th century cholera epidemics, and is also known as the father of (modern) epidemiology. He began by noticing the significantly higher death rates in two areas supplied by Southwark Company. His identification of the Broad Street pump as the cause of the Soho epidemic is considered the classic example of epidemiology. He used chlorine in an attempt to clean the water and had the handle removed, thus ending the outbreak. This has been perceived as a major event in the history of public health and regarded as the founding event of the science of epidemiology, having helped shape public health policies around the world. However, Snow’s research and preventive measures to avoid further outbreaks were not fully accepted or put into practice until after his death.

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Snow cholera map: A variant of the original map drawn by Dr. John Snow (1813-1858), a British physician who is one of the founders of medical epidemiology, showing cases of cholera in the London epidemics of 1854, clustered around the locations of water pumps.

In the early 20th century, mathematical methods were introduced into epidemiology by Ronald Ross, Anderson Gray McKendrick and others. Another breakthrough was the 1954 publication of the results of a British Doctors Study, led by Richard Doll and Austin Bradford Hill, which lent very strong statistical support to the suspicion that tobacco smoking was linked to lung cancer.

The Science of Epidemiology

Epidemiological studies include disease etiology, disease surveillance and screening, biomonitoring, and clinical trials.

Learning Objectives

Discuss the various factors that characterize epidemiology

Key Takeaways

Key Points

  • Epidemiologists rely on other scientific disciplines like biology to better understand disease processes, statistics to make efficient use of the data and draw appropriate conclusions, social sciences to better understand proximate and distal causes, and engineering for exposure assessment.
  • Epidemiologists employ a range of study designs from the observational to experimental. Its study designs are generally categorized as descriptive, analytical, and experimental.
  • The identification of causal relationships between disease exposures and outcomes is an important aspect of epidemiology.

Key Terms

  • epidemiologist: A scientist (often a medical doctor) who specializes in epidemiology.
  • causal: A cause of something; causing.

Major areas of epidemiological study include disease etiology, outbreak investigation, disease surveillance and screening, biomonitoring, and comparisons of treatment effects such as in clinical trials. Epidemiologists rely on other scientific disciplines like biology to better understand disease processes, statistics to make efficient use of the data and draw appropriate conclusions, social sciences to better understand proximate and distal causes, and engineering for exposure assessment.

Epidemiological studies are aimed, where possible, at revealing unbiased relationships between exposures such as alcohol or smoking, biological agents, stress, or chemicals to mortality or morbidity. Epidemiologists employ a range of study designs from the observational to experimental. Its study designs are generally categorized as descriptive, analytical (aiming to further examine known associations or hypothesized relationships), and experimental (a term often equated with clinical or community trials of treatments and other interventions).

In observational studies, nature is allowed to “take its course”, as epidemiologists observe from the sidelines. Observational studies have two components: descriptive or analytical. Descriptive observations pertain to the “who, what, where and when of health-related state occurrence”. On the other hand, analytical observations deal more with the “how” of a health-related event.

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World map of people living with HIV/AIDS: This map captures the estimated number of people in the world living with HIV/AIDS in 2008.

Controversially, in experimental studies, the epidemiologist is the one in control of all of the factors relating to the particular case study. Experimental epidemiology contains three case types: randomized control trials (often used for new medicine or drug testing), field trials (conducted on those at a high risk of conducting a disease), and community trials (research on social originating diseases).

The identification of causal relationships between these exposures and outcomes is an important aspect of epidemiology. It is nearly impossible to say with perfect accuracy how even the most simple physical systems behave beyond the immediate future. The complex field of epidemiology, which draws on biology, sociology, mathematics, statistics, anthropology, psychology, and policy only makes analysis even more challenging.

A common theme in much of the epidemiological literature is that “correlation does not imply causation. ” For epidemiologists, the key is in the term inference. Epidemiologists use gathered data and a broad range of biomedical and psychosocial theories in an iterative way to generate or expand theory, to test hypotheses, and to make educated, informed assertions about which relationships are causal, and about exactly how they are causal.

The Vocabulary Epidemiology

Epidemiology is the study of the patterns, causes, and effects of health and disease conditions in set populations.

Learning Objectives

Compare and contrast the following concepts: epidemic, endemic, pandemic; incidence vs prevalence; morbidity vs mortality; incubation, latency, acute, decline and convalescent periods

Key Takeaways

Key Points

  • Outbreak is a term used in epidemiology to describe an occurrence of disease greater than would otherwise be expected at a particular time and place.
  • An asymptomatic carrier is a person or other organism that has contracted an infectious disease, but who displays no symptoms.
  • Diseases that are “visited upon” a population are epidemic, whereas those that “reside within” a population are endemic. A pandemic is an epidemic of infectious disease that has spread through human populations across a large region.
  • Incidence is a measure of the risk of developing some new condition within a specified period of time. Prevalence is a measure of the total number of cases of disease in a population.
  • Morbidity is a diseased state, disability, or poor health due to any cause. The mortality rate of a condition is the proportion of people dying from it during a given time interval.
  • The progression of an infection usually follows these phases: infection, incubation period, acute period, decline period, and convalescent period.

Key Terms

  • epidemiology: The branch of a science dealing with the spread and control of diseases, computer viruses, concepts, etc., throughout populations or systems.

The Study of Disease

Epidemiology, literally meaning “the study of what is upon the people”, is derived from Greek: epi, meaning “upon, among”, demos, meaning “people, district”, and logos, meaning “study, word, discourse”, suggesting that it applies only to human populations. However, the term is widely used in studies of zoological populations (veterinary epidemiology) and of plant populations (botanical or plant disease epidemiology).

Outbreak is a term used in epidemiology to describe an occurrence of disease greater than would otherwise be expected at a particular time and place. It may affect a small and localized group or impact upon thousands of people across an entire continent. An asymptomatic carrier (healthy carrier or just carrier) is a person or other organism that has contracted an infectious disease, but who displays no symptoms. Although unaffected by the disease themselves, carriers can transmit it to others. A number of animal species act as vectors of human diseases.

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Mary Mallon: Mary Mallon (1870-1938) was nicknamed “Typhoid Mary,” an asymptomatic carrier of typhoid fever. She worked as a cook for several families in New York City at the beginning of the 20th century and infected many of them with typhoid. Note that in this 1909 newspaper illustration, she casts skulls into the skillet. However, this drawing inaccurately depicts the spread of typhoid, which was not by breathing, but by direct contamination from fecal particles.

EPIDEMIC, ENDEMIC OR PANDEMIC?

The distinction between “epidemic” and “endemic” was first drawn by Hippocrates, to distinguish between diseases that are “visited upon” a population (epidemic) from those that “reside within” a population (endemic). A pandemic is an epidemic of infectious disease that has spread through human populations across a large region; for instance multiple continents, or even worldwide. The term epidemiology is now widely applied to cover the description and causation of not only epidemic disease, but of disease in general, and even many non-disease health-related conditions, such as high blood pressure and obesity.

INCIDENCE VS. PREVALENCE

Incidence is a measure of the risk of developing some new condition within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during a time period, it is better expressed as the incidence rate which is the number of new cases per population in a given time period. Incidence should not be confused with prevalence, which is a measure of the total number of cases of disease in a population rather than the rate of occurrence of new cases. Thus, incidence conveys information about the risk of contracting the disease, whereas prevalence indicates how widespread the disease is. Prevalence is the proportion of the total number of cases to the total population and is more a measure of the burden of the disease on society.

MORBIDITY VS. MORTALITY

Morbidity is a diseased state, disability, or poor health due to any cause. The term may be used to refer to the existence of any form of disease, or to the degree that a health condition affects the patient. In epidemiology, the term morbidity rate can refer to either the incidence rate, or the prevalence of a disease, or medical condition. This measure of sickness is contrasted with the mortality rate of a condition, which is the proportion of people dying during a given time interval.

PHASES OF DISEASES

Epidemiologists are interested in determining the progression of a disease. In an infectious disease, the incubation period is the time between infection and the appearance of symptoms (acute period). The latency period is the time between infection and the ability of the disease to spread to another person, which may precede, follow, or be simultaneous with the appearance of symptoms. In most illnesses, the acute period is followed by the decline period (symptoms get better) and convalescent (or recovery) period.

Some viruses also exhibit a dormant phase, called viral latency, in which the virus hides in the body in an inactive state. For example, varicella zoster virus causes chickenpox in the acute phase; after recovery from chickenpox, the virus may remain dormant in nerve cells for many years, and later cause herpes zoster (shingles).

Koch’s Postulates

Koch’s postulates are four criteria designed in the 1880’s to establish a causal relationship between a causative microbe and a disease.

Learning Objectives

List Koch’s postulates

Key Takeaways

Key Points

  • The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884 and refined and published by Koch in 1890.
  • Postulate 1: The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.
  • Postulate 2: The microorganism must be isolated from a diseased organism and grown in pure culture.
  • Postulate 3: The cultured microorganism should cause disease when introduced into a healthy organism.
  • Postulate 4: The microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

Key Terms

  • Koch’s postulates: four criteria designed to establish a causal relationship between a causative microbe and a disease
  • postulate: A fundamental element; a basic principle.

Koch’s postulates are four criteria designed to establish a causal relationship between a causative microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884 and refined and published by Koch in 1890. Koch applied the postulates to establish the etiology of anthrax and tuberculosis, but they have been generalized to other diseases.

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Robert Koch: Robert Koch circa 1900. Koch’s postulates are four criteria designed in the 1880’s to establish a causal relationship between a causative microbe and a disease.

Koch’s postulates are the following:

  1. The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.
  2. The microorganism must be isolated from a diseased organism and grown in pure culture.
  3. The cultured microorganism should cause disease when introduced into a healthy organism.
  4. The microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

Koch’s postulates were developed in the 19th century as general guidelines to identify pathogens that could be isolated with the techniques of the day. Even in Koch’s time, it was recognized that some infectious agents were clearly responsible for disease even though they did not fulfill all of the postulates. Attempts to rigidly apply Koch’s postulates to the diagnosis of viral diseases in the late 19th century, at a time when viruses could not be seen or isolated in culture, may have impeded the early development of the field of virology. Currently, a number of infectious agents are accepted as the cause of disease despite their not fulfilling all of Koch’s postulates. Therefore, while Koch’s postulates retain historical importance and continue to inform the approach to microbiologic diagnosis, fulfillment of all four postulates is not required to demonstrate causality.

Koch’s postulates have also influenced scientists who examine microbial pathogenesis from a molecular point of view. In the 1980s, a molecular version of Koch’s postulates was developed to guide the identification of microbial genes encoding virulence factors.

Exceptions to Koch’s Postulates

Even in Koch’s time, it was recognized that infectious agents could be responsible for disease without fulfilling all of the postulates.

Learning Objectives

Recognize the exception to Koch’s postulates

Key Takeaways

Key Points

  • Koch abandoned the requirement of the first postulate altogether when he discovered asymptomatic carriers of cholera.
  • The second postulate may also be suspended for certain microorganisms or entities that cannot (at the present time) be grown in pure culture, such as prions responsible for Creutzfeldt–Jakob disease.
  • The third postulate specifies “should”, not “must”, because as Koch himself proved in regard to both tuberculosis and cholera, not all organisms exposed to an infectious agent will acquire the infection.

Key Terms

  • asymptomatic: not exhibiting any symptoms of disease.

Koch’s postulates were developed in the 19th century as general guidelines to identify pathogens that could be isolated with the techniques of the day. Even in Koch’s time, it was recognized that some infectious agents were clearly responsible for disease, even though they did not fulfill all of the postulates.

Currently, a number of infectious agents are accepted as the cause of diseases despite their not fulfilling all of Koch’s postulates. Therefore, while Koch’s postulates retain historical importance and continue to inform the approach to microbiologic diagnosis, fulfillment of all four postulates is not required to demonstrate causality.

Koch abandoned the requirement of the first postulate altogether when he discovered asymptomatic carriers of cholera and, later, of typhoid fever. Asymptomatic or subclinical infection carriers are now known to be a common feature of many infectious diseases, especially viruses such as polio, herpes simplex, HIV, and hepatitis C. Specifically, all doctors and virologists agree that the poliovirus causes paralysis in just a few infected subjects, and the success of the polio vaccine in preventing disease supports the conviction that the poliovirus is the causative agent.

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Cholera bacteria: Scanning electron microscope image of Vibrio cholerae bacteria, which infect the digestive system.

The second postulate may also be suspended for certain microorganisms or entities that cannot (at the present time) be grown in pure culture, such as prions responsible for Creutzfeldt–Jakob disease.

The third postulate specifies “should”, not “must”, because as Koch himself proved in regard to both tuberculosis and cholera, that not all organisms exposed to an infectious agent will acquire the infection. Noninfection may be due to such factors as general health and proper immune functioning; acquired immunity from previous exposure or vaccination; or genetic immunity, as with the resistance to malaria conferred by possessing at least one sickle cell allele.

In summary, a body of evidence that satisfies Koch’s postulates is sufficient but not necessary to establish causation.