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Science of Epidemiology

Epidemiology is the study of the cause, transmission, timing, and distribution of infectious disease episodes, with a focus on recognizing outbreaks, controlling those outbreaks, and treating the infected. Epidemiologists count cases of a particular disease (location, date, people involved) and determine disease rate.

Epidemiology is the science that investigates the cause, transmission, timing, and distribution of infectious disease episodes, with a focus on recognizing outbreaks, controlling those outbreaks, and treating the infected. Etiology is the study of the disease causes. Epidemiology is the foundation of monitoring and maintaining good public health. Since epidemiology identifies the risk factors for various diseases, it is vital for shaping policy decisions regarding public health. This information can then be used to prevent further outbreaks.

Modern epidemiology began in the mid-1800s with the work of British physician (1813 – 58) Dr. John Snow. Dr. Snow, a British physician, investigated the 1849–54 outbreak of cholera in London. Snow analyzed records of deaths caused by cholera, interviewed survivors who lived in the neighborhood where the deaths occurred, and gathered information about the victims. He made a map showing that most people who died of cholera drank or bought water from a certain water pump. The contaminated water, he concluded, caused the cholera outbreaks.

John Snow's Map of Fatal Cholera Cases

In 1855, British physician John Snow published the mapped out locations of patients who died of cholera in London. By analyzing the pattern of cholera cases and interviewing survivors in the neighborhood, Dr. Snow was able to identify the contaminated pump containing the tainted water that caused those deaths.
Credit: John Snow - Published by C.F. Cheffins, Lith, Southhampton Buildings, London, England, 1854
Another early pioneer of epidemiology was Hungarian physician Dr. Ignaz Semmelweis (1818 – 65). Dr. Semmelweis recorded the number of births and deaths at Vienna General Hospital from 1846 to 1848. Through this record keeping, he discovered a trend in maternal deaths. There was a higher death rate because of puerperal sepsis (childbirth fever) at this particular hospital than for people who did not go to this hospital. It was found that medical students who had spent the morning dissecting cadavers were examining the patients at this hospital. The students were ordered to wash their hands and the death rates dropped significantly. In 1854, English nurse Florence Nightingale was commissioned to form a corps of nurses to attend the sick and fallen soldiers of the Crimean War (October 1853 – February 1856). Nightingale and her team discovered that due to a short supply of medication and poor hygiene, mass fatal infections such as typhus, typhoid, and cholera were common among the soldiers. In order to improve conditions for the infirm, Nightingale developed visualization techniques to chart or plot the various causes of the soldiers’ deaths over time. One visualization method invented by Nightingale is the Coxcomb chart, also called a rose chart or polar area diagram. By utilizing this technique, Nightingale was able to discover that more soldiers’ deaths were the result of contamination conditions in the hospitals were they were being treated, rather than the result of battle wounds. Consequently, Nightingale and her fellow nurses instituted a regime of better sanitation and hygiene, including hand washing, in the hospital setting. These practices reduced the soldiers’ death rate from 42% (during the time of her arrival in 1854) to 2% at the end of the war in 1856.

Florence Nightingale's Coxcomb Chart

Florence Nightingale designed visualizations with data on the deaths of British soldiers during the Crimean War of 1854-56. These visualizations persuasively communicated that most deaths were due to preventable diseases.
The work of John Snow, Ignaz Semmelweis, and Florence Nightingale examined when and where diseases occurred and how they happened. An epidemiologist determines the cause of a disease and looks at patterns and factors about the people affected. They also look at various methods for controlling a disease through preventative strategies:
  • Primordial prevention—disease prevention at an early stage before risk factors are present in the population (e.g., advising parents to stop smoking to teach their children that smoking is a dangerous habit that should be avoided)
  • Primary prevention—disease prevention by limiting risk factor exposure (e.g., preventing heart disease by encouraging weight loss and exercise in obese patients)
  • Secondary prevention—employment of screening measures to detect disease and treat it in the early stages (e.g., performing mammograms to detect potential breast cancer)
  • Tertiary prevention—monitoring of patients with long-term diseases to ensure maintenance of health (e.g., regular blood glucose checks for patients with diabetes, or cholesterol checks for patients with hypercholesterolemia and family risk of heart disease)

The germ theory of disease states that diseases are caused by microorganisms. This is the currently accepted scientific theory of today, but this was not always the case. Historically, disease, and the “germs” causing disease, were thought to occur spontaneously. In the early 1860s, French biologist, microbiologist, and chemist Louis Pasteur began conducting experiments to ascertain the relationship between pathogens and disease. He boiled broth and exposed the broth to air filtered to prevent particles from passing through to the broth. No growth occurred in the broth, therefore disproving the theory the microorganisms spontaneously appeared. In 1890, German physician and microbiologist Robert Koch published four criteria, known as postulates, for demonstrating that disease is, in fact, caused by an organism.

Today epidemiologists use information from a variety of disciplines to track, predict, and prevent outbreaks. Health care professionals, immunologists, and microbiologists may all be consulted in an effort to identify the causative agent. Epidemiologists describe the occurrence, frequency, and distribution of a disease by the incidence, prevalence, morbidity rate, mortality rate of the outbreak, which may be classified as endemic, epidemic, hyperendemic, sporadic, and pandemic. Epidemiology examines disease frequency related to the population at risk. The population at risk can be as small as a single family or as large as the worldwide population of people. For any disease, a series of statistics can be measured, which helps epidemiologists to characterize the disease. The disease's incidence is the number of new disease cases recorded during a certain period of time. A disease's prevalence is the total number of cases within a population during a certain period of time. The morbidity rate is the frequency of a disease in a certain population. Morbidity means a deviation from health; therefore, all diseases have a morbidity rate. Lastly, diseases have a mortality rate. The mortality rate is the frequency of deaths in a population. Some diseases have very low mortality rates, for example, varicella. Also called chicken pox, the mortality rate for varicella is 0.02%. Rabies is a disease with a very high mortality rate—100%, in fact. Although rabies is preventable with postexposure prophylaxis, once symptoms begin, all infected organisms die.

Summary of Terms of Disease Statistics

Term Definition
Incidence Number of new disease cases recorded during a certain period of time
Morbidity rate Frequency of a disease in a certain population
Mortality rate Frequency of deaths in a population
Prevalence Total number of cases within a population during a certain period of time