Epidemiology is the science dealing with the spread and control of diseases and other factors relating to health in populations and other groups.
Epidemiological information is used to identify and evaluate the risks of epidemic outbreaks of disease and to help prevent their spread or return.
What do epidemiologists do?
Epidemiology relates to specific populations as well as specific diseases. Epidemiologists do a number of things to investigate both the health threat and the population affected:
They collect and interpret data about incidences of, for example, a particular disease and the people who have it. The data used can be statistics such as mortality rates and incidences of disease. It can also be in the form of surveys designed to gather information about a population, such as their lifestyle or ethnicity.
Some statistics are gathered from the whole of the target population, but more often observations can only be made on a study sample, which is selected from the target population.
The sample is chosen at random from a study population, which can be identified on the basis of location, occupation, age, sex, lifestyle, diagnosis or treatment—or some combination of these.
Such studies can provide clues to the cause and mode of transmission of a disease. For example, comparing incidences of lung cancer in smokers and non-smokers showed a correlation between smoking and lung cancer.
Epidemiologists also monitor trends over time. These longitudinal studies can identify emerging health issues and assess the effectiveness of control measures, such as vaccination.
The Ancient Greek physician Hippocrates was among the first to examine the relationship between disease and the environment and made the distinction between endemic diseases—those that constantly 'live' in a particular region—and epidemic diseases, which 'visit' a population and spread rapidly within the it.
Early attempts to track the spread of epidemics like the Black Death prompted authorities to gather information such as mortality statistics. But many elements of the science of epidemiology only emerged in the context of industrialisation and urbanisation during the 1800s as public health measures aimed at countering infectious diseases such as cholera.
Death data: the bills of mortality
Numerical data is collected as evidence for the existence and extent of disease outbreaks. It can also show connections and correlations between environmental factors, such as living conditions and lifestyles, and the health of a population.
One of the earliest examples of data gathering in relation to disease was the so-called bills of mortality—the weekly mortality statistics produced in London during outbreaks of the plague. Data was collected in an attempt to see if there was a pattern to the onset and spread of plague in the city.
The bills of mortality were produced intermittently until 1595 and then continuously from 1603 by the Worshipful Company of Parish Clerks. Over the next two centuries they added details of baptisms, cause of death and the age at death to their records. But because the data was collected by the parishes of the Church of England, the records did not include members of other churches and faiths, or non-church goers.
In 1662, John Graunt, a London haberdasher and councilman, published an analysis of data from the bills of mortality. In his book, Natural and Political Observations Made upon the Bills of Mortality (1662), he quantified patterns of birth, death and disease occurrence. He noted disparities between men and women, high infant mortality, urban/rural differences and seasonal variations.
Graunt, along with his friend William Petty, developed early human statistical and census methods such as life tables. These were tables of statistics relating to life expectancy and mortality for a given category of people.
Production of the bills went into decline from 1819 as parishes gradually stopped providing returns. They were superseded by the weekly returns of a government agency, the General Register Office, from 1840.
The Victorian period saw the growing importance of health-related statistics. Leading public health reformer Edwin Chadwick was at the forefront, gathering information through surveys and data collection to compile his Report on the Sanitary Conditions of the Labouring Population (1842). Statistics in the report showed just how unhealthy industrial towns and cities were, even for their wealthier inhabitants.
A few years earlier, the General Register Office (GRO) for England and Wales had been established in 1837. It was the government department responsible for recording births, deaths and marriages and for conducting the national census.
William Farr (1807–83), a London physician, laid much of the groundwork for the systematic collection and analysis of medical statistics at the GRO. Such detailed information provided the raw data for a detailed analysis of death within the general population. For example, the mortality rates of different professions or of those living in different locations could be compared and contrasted.
Farr was also a member of the Statistical Society and was familiar with the latest statistical techniques, such as the Gompertz curve and the law of mortality—a means to predict future mortality rates in a population from a sample of collected data. From the GRO statistics he constructed a series of national life tables.
Another important figure in public health in London around that time was Dr John Simon (1816–1904). Simon was London's first Medical Officer of Health, part of a trend that saw public health administration shift from social reformers to medical doctors. He went on to become the Chief Medical Officer, the government's main adviser on health matters. It's a role that still exists in the United Kingdom today.
Although he recognised the significance of social and environmental issues in public health, Simon also saw a role for the state in the investigation of infectious diseases such as diphtheria, typhoid and smallpox. He oversaw the creation of a state system of medicine that used epidemiological as well as laboratory science in the control and prevention of disease.
He produced a Europe-wide survey on the effectiveness of different vaccination practices, which helped to persuade parliament to introduce compulsory smallpox vaccination in England and Wales in the 1853. This legal move probably did more to encourage the growth of an anti-vaccination movement than any health concerns about vaccines.
Florence Nightingale: The pioneering statistician
Florence Nightingale also set about collecting statistics during the Crimean War (1853–6). She recorded the causes of soldiers’ deaths over two successive years in the Crimea and was able to show that more soldiers died from preventable diseases during the war than from injuries.
When Nightingale returned from the Crimea to London in 1856 she set about publicising her statistical findings as well as her proposed medical reforms. Nightingale was elected the first female member of the Statistical Society (now the Royal Statistical Society) in 1858, just two years after returning from the Crimea.
Find out more in our story Florence Nightingale: The pioneering statistician
An early epidemiological investigation of cholera
Dr John Snow's investigations into the mid-century outbreaks of cholera in London contain many of the elements of contemporary epidemiological studies, although their influence on public health policy at the time was limited.
During earlier outbreaks, Snow had formed the opinion that cholera was spread through contaminated water rather than by airborne miasmas—a once popular theory that diseases were caused by poisonous vapours characterised by their foul smell. He published his findings in a paper entitled On the Mode of Transmission of Cholera (1849), but failed to convince most of his medical colleagues.
But during the 1854–55 cholera outbreak in Soho, London, he set out to prove his theory with a number of investigations of the people who contracted the disease and their location in the city. He began by gathering data from local hospital and public records of all the incidences of cholera and questioned the households where the disease occurred. He then marked his findings on a map of the Soho area.
Because Snow believed that contaminated water was the source of infection in this outbreak, he also marked the location of public water pumps on his map. His map showed a correlation between the distribution cases of cholera and the location of one particular pump.
The map confirmed what cholera survivors had told him about where they had got their drinking water. Based on the distribution of cases, he proposed that a pump on Broad Street was the most likely source of cholera infection in the area.
His map showed up other supporting evidence for his theory about the transmission of cholera. Having noted that there were no cases of the disease in a small area just to the east of the Broad Street pump, upon further investigation Snow found a brewery located there with a deep well on the premises. The locals and brewery workers got their water from this well, along with a daily portion of malt liquor. This would account for the absence of cholera infections around the brewery.
Snow showed his evidence to the local authorities and finally persuaded them to remove the handle from the Broad Street pump, although the outbreak was already petering out by that stage.
The 1854–55 outbreak was also investigated by the General Board of Health's 1854 Committee for Scientific Enquiries, which included William Farr. The findings of the committee were that the cause of cholera was multifactorial. They acknowledged Snow's evidence from Broad Street, but concluded that contaminated water was one factor in the spread of cholera, along with miasmas and the low elevation of much of London.
There was a further outbreak of cholera in 1866, by which time Snow had died. But William Farr, who had initially been reluctant to accept Snow's conclusions, was finally convinced. His own data from the new outbreak showed that mortality was extremely high among those drawing their water from the Old Ford Reservoir in East London, one of the last areas in the capital without a clean water supply.
The epidemiology of chronic diseases
As many of the endemic infectious diseases were controlled by mass vaccination and improved sanitation in the 20th century, epidemiologists extended their methods to investigate non-infectious diseases.
Epidemiology has been applied to a wide range of health-related outcomes, behaviours and even knowledge and attitudes. The studies by British researchers Richard Doll and Austin Bradford Hill that linked lung cancer to smoking are an example of how epidemiological methods have been applied to lifestyle choices. Their key 1954 publication lent very strong statistical support to the growing evidence that tobacco smoking posed a significant danger to health.
As well as diseases linked to lifestyle and occupation, epidemiologists have turned their attention to chronic diseases such as heart disease and diabetes. And growing knowledge about genetics and epigenetics have given them new ways to analyse possible risk factors in childhood, rather than solely concentrating on risk factors in adulthood, such as smoking, diet and lack of physical exercise.
Epidemiology is one of the few sciences in which the past is as important as the future. Longitudinal (time-based) studies can demonstrate how a disease interacts with different populations and give public health officials clues about how it tackle it in the future.
However, as the limited scope of the mortality bills demonstrates, one of the most difficult aspects of using historical data is the standard and reliability of data collected in the past. The same issue applies when comparing data sets from different countries and different medical authorities.
Epidemiologists continue to monitor outbreaks of disease across the world, on the lookout for a new strain of influenza or another deadly outbreak of diseases such as Ebola. And that's not easy, as science writer Leigh Cowart says:
To be prepared for Ebola we need to discover how the virus moves through the wild and the city alike. We must find out where it thrives and how it spills over, and we must track it to all of the places it goes when we are not watching from the hospital bedside.
Suggestions for further research
- L Berkman, I Kawachi and M Glymour (Editors), Social Epidemiology (New York: Oxford University Press, 2014)
- A Broadbent, Philosophy of Epidemiology (London: Palgrave Macmillan, 2013)
- S Hempel, The Atlas of Disease: Mapping deadly epidemics and contagion from the plague to the zika virus (London: White Lion Publishing, 2018)
- S Johnson, The Ghost Map: The Story of London's Most Terrifying Epidemic—And How It Changed Science, Cities, and the Modern World (New York: Riverhead Books, 2006)
- N Krieger, Epidemiology and the People's Health: Theory And Context (New York: Oxford University Press, 2011)