Controlling Outbreaks

In this series of public health articles, you will be able to learn about the fundamentals of public health which will serve you well for your medical school interviews. These principles will help you to understand how medicine and public health are intertwined especially as a result of the Coronavirus pandemic.

Throughout history we have witnessed many outbreaks including Influenza, Cholera, smallpox, MERS, Ebola and now the latest one COVID-19. Back in the times of the black death, causes, prevention and management methods of infectious diseases were unknown and without this, doctors were left powerless to stop outbreaks. It was widely believed that infectious diseases were caused by movement of the planets, punishment from God, bad smells & corrupt air (miasma theory) and many more such theories. The concept of epidemiology was unknown until John Snow- the father of modern epidemiology.


Controlling outbreaks follow a logical public health approach consisting of 4 general steps: identifying an outbreak, surveillance, epidemiological investigation, and control & communication. We can use the very well-known case study of John Snow and cholera to highlight how this public approach came to be and formed the foundation for epidemiology. The 1854 cholera outbreak in London yet again stirred up ideas of bad smells and miasma, however, John Snow – a London based physician – rightly proposed that the outbreak was not transmitted by bad air but by water-borne infection. Unfortunately, his ideas were not well received at first which is when he decided he would have to undergo proper epidemiological methods to prove his ideas.

🔍Types of Surveillance

Once an outbreak is identified through reports from clinicians, patients, public health officials, media etc, effective surveillance is key. There are two main types of surveillance: passive and active. The former refers to routine systems which rely on clinicians remembering to send in case reports or case identification from laboratories. The latter is where a surveillance team actively seeks out reports and conducts tracing procedures, this is very resource intensive. Back in the 1800s there were not enough resources for active surveillance, therefore, surveillance was usually based on case reports from clinicians, however, more often it was based on the death reports as in the case of John Snow who recorded deaths related to cholera in the area. Today we have specific surveillance teams from different organisations such as European Centre for disease prevention and control (ECDC), WHO, Public Health England etc. The purpose of effective surveillance is to monitor the burden and distribution of disease, detect the occurrence of outbreaks and evaluate impact of control and prevention activities.


Once an efficient surveillance system is set up, the next step in an outbreak is to undertake epidemiological investigation which involves collecting information about cases and non-cases and looking for links between the cases. This can be done through questionnaires and collecting information about the cases such as age, sex, occupation, travel etc and more specific questions about contact history, food diaries, location of home/work etc. Generating an epidemic curve allows identification of the source as you can see a graphical display of the progression of the outbreak and how this might link to different sources. A scientific hypothesis is then generated which can be tested through cohort or case-control studies.

🦠Cholera Case Study

Let us look at the cholera case study and how this epidemiological investigation was carried out by John Snow. Snow initially investigated the cases and concluded that disease must be spread by water or food consumption because the victims had symptoms related to the gastrointestinal tract. Therefore, the initial opinion by the majority that this was caused by bad air did not seem likely as that should have caused pulmonary symptoms. This is a great example of surveillance, case identification and case definition. He then carried out epidemiological investigation by conducting questionnaires of the first 83 victims and learned that the majority of them collected water from one particular water pump. This led him to generate a hypothesis that drinking water from this pump is associated with getting infected with cholera.

After carrying out informal studies that resemble case-control studies as we now know it, Snow was able to prove his hypothesis and persuaded the local Board of Guardians to remove the pump handle after which deaths from cholera significantly reduced.

It was later found that the water pump was contaminated with faeces from an infant who had died from diarrhoea at the beginning of the outbreak.

In retrospect, we can now see that Snow made several important contributions to epidemiology as we now know it by carrying out surveillance, forming an hypothesis, testing this hypothesis by making comparisons between groups of people, providing the evidence for association, implementing interventions to control the outbreak, and again carrying out surveillance to assess the impact of the intervention. This is the public health approach to controlling outbreaks and it uses core public health disciplines within epidemiology. Understanding this process is vital for medics and doctors to effectively participate in understanding and controlling outbreaks.

📜 Further Reading


By Nafisa Insan BSc MPH

PhD Student in Population Health Sciences

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