The Science of Public Health

You know Jon Snow, right?

No, not the Jon Snow whose watch has ended. I’m talking about the John Snow who took the handle off a water pump in the middle of London and stopped a cholera epidemic in it’s tracks.

This John Snow is considered the Father of Epidemiology.

In 1849, the prevailing theory was that cholera (a diarrheal illness) was spread by miasmas – “bad air” – or person to person contact. Snow published a paper that presented his theory of disease — acquisition by ingestion of contaminated water — but it was not well received. He continued to collect data and found evidence that linked cholera to specific water sources. 

A cholera outbreak in 1854 in the Broad Street area of London piqued John Snow’s suspicion. He obtained the names and addresses of the first 83 victims who died in the first week and found that the majority of them obtained their water from the Broad Street water pump. Snow was able to convince the local governing council to remove the pump handle and the outbreak quickly subsided, proving the his theory of cholera transmission through contaminated water.

John Snow used maps and records to track disease spread back to its source. His ideas provided the foundation for how we find and stop disease today – the science of epidemiology.

What is Epidemiology?

Epidemiology is the science of public health. It is used to find the causes of health outcomes and diseases in populations. It looks at distribution (frequency, pattern) and determinants (cause, risk factors) of health-related events (not just diseases!) in specific populations (neighborhood, school, city, state, country, global). The types of problems investigated with epidemiology include environmental exposures like lead or air pollutants; infectious diseases such as food borne illnesses or COVID-19; injuries from homicides to car accidents; non-infectious diseases like cancer or birth defects; natural disasters; and terrorism related activities like anthrax. Epidemiologists gather data and use statistical analysis to find our how a particular health problem was introduced and make recommendations to control the spread or prevent future events.

Here are some important epidemiology terms to know and how they relate to COVID-19.

Sensitivity is the ability of a system to detect epidemics and other changes in disease occurrence. It is proportion of persons with disease who are correctly identified by a screening test or case definition as having disease. A highly sensitive COVID-19 test will correctly identify those who are positive for the disease – the true positives.

Specificity is the proportion of persons without disease who are correctly identified by a screening test or case definition as not having disease. A highly specific COVID-19 test will correctly identify those who are negative for the disease – the true negatives.


So how sensitive and specific are the COVID-19 tests currently being used? The RT-PCR test (the swab up the nose to collect bits of viral RNA) has high sensitivity and high specificity. This is an ideal test in epidemiological terms, however a negative PCR test does not mean you are 100% a true negative – it means you are a true negative in the exact moment of the test. You could still become infected after the test is performed or there is the chance you just don’t have enough viral material for the test to pick up the virus yet. The antigen test (the swab up the nose to collect viral particles) has moderate sensitivity and high specificity. This means it is better at ruling disease out than ruling disease in – i.e. you can trust a negative test a bit more than a positive test.

Incidence is measure of the frequency with which an event, such as a new case of illness, occurs in a population over a period of time. The denominator is the population at risk; the numerator is the number of new cases occurring during a given time period. In terms of COVID-19, the denominator (population at risk) would be the entire U.S. population, although a case could be made to exclude anyone who has already been infected because the risk of re-infection is rare. The number of new cases is exactly as it sounds – new infections in a specific time period. With COVID-19, this requires some estimation because for every case that is diagnosed with a test, there are many who never get tested.

Prevalence is number of cases or events or conditions in a given population at a specific point in time. For COVID-19, this would be all the cases of the disease, not just new cases.

Case Fatality Rate is the proportion of persons with a particular condition (cases) who die from that condition. The denominator is the number of incident cases; the numerator is the number of cause-specific deaths among those cases. According to the latest data for the United States, there have been 266,094 deaths from COVID-19 and 13,244,417 cases. This translates to a case fatality rate (or a mortality rate) of 2.0%. While other countries have a higher case fatality rate than the United States (Mexico is 9.6%, Ireland is 2.8%, etc.), they have both far fewer cases and far fewer deaths than the U.S. You can view that information here.

Who does the work of epidemiology?

Epidemiologists work at public health organizations (CDC, WHO), in local and state health departments, in private industry, and at academic and medical institutions. The work of epidemiologists often provides evidence to policymakers about how the environment, technology, medical practice, or other social determinants affect population health. Within the CDC, the Epidemic Intelligence Service (EIS) have served as the front line of public health for over 60 years. They are “disease detectives,” acting as boots on the ground tracing and responding to threats like polio, smallpox, Ebola, HIV/AIDS, Zika, and now COVID-19. 

Add The Ghost Map to your TBR pile to learn more about John Snow and his work as a disease detective.

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