Learn all about the study of diseases in populations, what the difference between endemic, epidemic and pandemic is, and how we identify the important factors.


Epidemiology is the study of diseases in populations, and it examines how, where, and why they occur. The World Health Organization defines epidemiology as:

*“The study of the distribution and determinants of health-related states or events (including disease), and the application of this study to the control of diseases and other health problems.”*

The Greek physician Hippocrates, known as the father of medicine, observed the impact of environmental factors on the occurrence of disease, and is considered to be the first epidemiologist. It may not be surprising that the term ‘epidemiology’ is derived from the combination of the following Greek words: *Epi*, meaning ‘upon, among;’ *demos*, meaning ‘people, district;’ and *logos*, meaning ‘word, study.’

Hippocrates also made the distinction between *epidemic* and *endemic*, distinguishing between diseases that ‘visit upon’ a population (epidemic) from those that ‘reside within’ a population (endemic).

Girolamo Fracastoro

In the mid-1500s, an Italian physician named Girolamo Fracastoro proposed the first scientific germ theory of disease in *De contagione et contagiosis morbis* (On Contagion and Contagious Diseases) in 1546, around 300 years before it was empirically formulated by Louis Pasteur and Robert Koch.

Fracastoro suggested that each epidemic disease is caused by a different type of rapidly multiplying miniscule ‘seed-like entities’ that could be spread in 3 ways: by direct contact; by carriers such as dirty clothes; and through the air. Frascatoro was the first to scientifically explore and understand the true nature of contagion, infection, and disease transmission.


John Snow

Modern epidemiology didn’t become an established medical field until 1854 when a cholera outbreak swept the city of London. British doctor John Snow suspected that the cholera bug was being spread though polluted water and noticed that death rates were significantly higher in two areas that were supplied by the Southwark and Vauxhall Water Company.

Snow’s data collection and analysis was unparalleled. He carefully mapped out the outbreak by tracking down 83 victims and collecting details of where they had obtained their drinking water from, linking every single case to a water pump on Broad Street. Snow removed the pump handle as a precaution and the epidemic subsided quickly after that.

Snow remained unrecognized in his time but is today considered the father of modern epidemiology. His methods formed the basis of epidemiological practice in modern medicine and have helped shape public health policies around the world.


The Emergence of Germ Theory

Louis Pasteur was a French chemist and microbiologist, and the first to experimentally prove the connection between germs and disease in the 1860s. Pasteur’s conclusive experiments served to debunk the theory of *spontaneous generation* – essentially that life arises from non-living mater – that had been dominating the medical field from ancient times. His pioneering studies revolutionized the understanding of diseases and their cause, laying the foundation for modern day treatment.

With this shift in paradigm the focus of epidemiologists changed to the causes, consequences, treatment, and prevention of diseases. Following Robert Koch’s discovery that specific germs cause specific diseases, epidemiologists sought to determine the germs responsible for major diseases which, in turn, allowed them to focus on their transmission patterns.

Clinical Medicine vs Epidemiology

While epidemiology is an entirely different field from clinical medicine, both are closely related and work together in the continuous quest to understand, prevent and treat diseases. Epidemiology is different to clinical medicine as it focuses on infection and disease in a community rather than an individual. In clinical medicine, doctors are concerned with the disease pattern presenting in an individual patient, whereas epidemiologists are concerned with disease patterns in entire populations.

An important differentiation is that, while doctors are primarily concerned with disease, epidemiologists are interested in both infection and disease. It is possible to encounter infection that never develops into disease, and so studying clinical illness alone would give an unreliable epidemiological picture and lead to ineffective control and prevention. Similarly, epidemiologists study both the ill and the healthy to get a better understanding of patterns and causes, while doctors only deal with sick patients.

Classification of Diseases in Epidemiology

In epidemiology, researchers classify diseases based on their type and frequency of occurrence as being either *endemic* or *epidemic*. Hippocrates was the first to make this distinction in the 5th century BCE.

Endemic diseases are those that are present, contained, and remain fairly consistent within a region or group of people, such as malaria which is endemic to parts of Africa due to the higher temperatures that allow the *Anopheles* mosquito to thrive. In contrast, an epidemic is a sudden spike in occurrence of a disease within a population or region, like the Ebola virus which spread rapidly throughout West Africa in 2014.

An epidemic can give rise to a *pandemic* which is a sudden rapid outbreak of a disease affecting populations across different geographical regions, such as COVID-19. Scientists determine whether a disease has become a pandemic if it is spreading at a very fast rate with new cases daily.

It is important to note that the classification of diseases in epidemiology doesn’t describe their severity but rather their prevalence.

Incidence vs Prevalence

Disease occurrence is measured based on incidence rates and prevalence rates. The incidence rate represents the number of new cases in a population over a period of time, whereas the prevalence rate measures the total number of existing cases of a disease in a population at a given point in time.

Incidence describes the rate of manifestation of a particular disease in a population, while prevalence reflects how widespread a disease is in a population. Furthermore, the incidence rate is essential in evaluating disease control programs and can affect future decisions in medical care. The prevalence rate can indicate the medical and social burden of a disease within a geographic region, and it is useful mostly in with diseases of long duration.

Prevalence rates are dependent on incidence and duration of a disease, so if the incidence rate of a disease is low but the duration long, then the prevalence rate will be high compared to the incidence rate. Equally, if the prevalence of a disease is low due to short duration, then the prevalence rate will be small compared to the incidence rate.

Epidemiological Modelling

Mathematical modelling was introduced into the field of epidemiology in the 20th century by scientists Anderson Gray McKendrick and Janet-Leigh Claypon, and has since been fundamental in managing disease outbreaks and informing public health decisions.

Epidemiological models show how infectious diseases progress in an epidemic based on the behavior of the infectious pathogen and the population itself. Such models aim to predict how effective interventions can be by using a set of assumptions and statistics to establish relevant parameters. These models help predict growth and spread patterns of diseases and which interventions should be implemented or avoided, such as social distancing and mass vaccination.

The complexity of epidemiological models varies, but an important concept is that of *thresholds*, which are a set of values including vector density, contact number, and population size. These thresholds determine whether a disease will remain endemic or become an epidemic and also help determine the R0 number of a disease in a population. The R0 indicates how transmissible a disease is.

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