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Pandemics and the Epidemiological Triangle

    During a particularly bad wave of Covid-19 in his province, one Canadian premier declared to the assembled media, “Covid is evil.”

    The major problem with that statement is not just that it attributes moral agency to a virus, but that it ignores the lessons of past pandemics. As British anesthetist Dr. John Snow so eloquently demonstrated in his series of “Grand Experiments” in London, England between 1848 and 1854 during the third global cholera pandemic, pandemics thrive only on the potentially explosive interplay between a pathogenic agent, susceptible hosts, and a fertile physical and social environment. In the case of cholera in Victorian London, Snow confirmed his hypothesis that the devastating cholera outbreaks in the Soho District of London were due to pathogens (later identified as Vibrio bacteria by Pacini and Koch) being ingested from sewage-contaminated water by susceptible impoverished Londoners who were largely unable to flee London or avoid exposure due to social circumstances of the day. Snow astutely recognized that without the host and environmental factors that made his London district vulnerable to explosive spread of the disease, cholera would not have thrived there.

    This necessary agent-host-environment interaction, originally coined the “epidemiologic triad”, is now commonly referred to as the “epidemiologic triangle” (CDC, 2011). It is a simple yet inclusive causal conceptual framework that arguably applies as cogently to the Covid-19 pandemic as it does to cholera in the 19th century. Particular characteristics of each of the corners of the triangle, and the complex interplay between these factors, have led to the persistent and destructive expansion of the Covid-19 global pandemic.

    Figure of triangle with a word at each points: Agent, environment and host
    Figure. The Epidemiologic Triangle

    The corner of the triangle that continues to garner most attention in the Covid-19 pandemic is the agent, the SARS-CoV-2 virus. Indeed, this virus ticks off many of the requisite boxes for a potent pandemic agent. It is a tiny virus with no previously known specific treatment. It is spread and acquired via the respiratory system, meaning it can be highly transmissible, and indeed has turned out to be significantly more infectious than influenza. It can be transmitted before symptoms even start, and even by people who are infected but never develop symptoms. It is novel, meaning it is new to humans, and it can mutate to become more infectious and potentially more severe.

    But this highly transmissible virus would not have become a potent pandemic agent without susceptible hosts. The most vulnerable hosts, demographically, in the Covid-19 pandemic have been those of advanced age. We have seen that (before the vaccine rollouts) the very elderly may be about a thousand times more likely to die of it than the very young. This exponential increase in risk of mortality with age is different from many other diseases (for example influenza and salmonella), which tend to most heavily impact both the very young and the very old. Co-morbidities such as pre-existing heart disease, diabetes and obesity have also been found to be potent host vulnerabilities for Covid-19. Research is ongoing into other potentially important biological host factors that may explain some of the unpredictability of severe outcomes. But the most common host risk factor of all for Covid-19 outcomes is immunologic naivety; i.e. the fact that none of us had specific acquired immunity to this specific virus strain, as it was truly novel to human populations. Our immune systems can overly react to novel invaders, leading to severe and lasting organ damage due to the immune response itself, and it is this effect which causes the most severe symptoms and mortality due to Covid-19. In contrast, the relative sparing of seniors from illness during the 2009 H1N1 influenza pandemic was due to the fact that those born before the 1950’s were likely to have been previously exposed to the H1N1 subtypes of influenza that had been globally circulating ever since the H1N1 “Spanish Flu” pandemic of 1918-1920, until new H3N2 subtypes appeared and became predominant in the late 1950’s and 60’s. This fact dramatically illustrates the protective power of acquired specific immunity to a pathogen, in spite of the general “senescence” (or weakening) of our innate immune system as we age. In other words, pandemics have repeatedly demonstrated how “novelty means severity.”

    The third and perhaps most important – while under-acknowledged – corner of the epidemiologic triangle is the environment into which susceptible hosts were exposed to the infectious agent in the evolving Covid-19 pandemic. As we have seen, physical environmental factors such as distance from source, indoor vs. outdoors settings, ventilation, population density and crowding are critical for a virus which, alas, can be transmitted through the air. The importance of other physical environmental factors such as season, temperature, humidity, and proximity of humans to certain animal populations is yet to be fully elucidated, but will be eventually. But what has been once again made abundantly clear in the current pandemic is that the social environments in which the infectious agent encounters susceptible hosts ultimately determine if and how extensively a pandemic propagates. In North America, we have seen devastating impacts of the pandemic on racialized, marginalized and/or impoverished communities, low-wage occupations and sub-populations facing structural barriers which preclude the protective measures benefiting the more privileged (Jones et al, 2021). Globally, many low and middle-income countries continue to be much more heavily impacted by Covid-19 than their wealthier and more powerful counterparts. Disparities in the timely distribution and access to the very effective Covid-19 vaccines may subject these regions to further unnecessary misery in the months and years to come. 

    Summing things up: successes and failures in both preparation and response to the current pandemic reflect the extent to which all corners of the epidemiologic triangle were duly considered and addressed, applying lessons from previous pandemics while being open-minded to the novelty of the current context. In the future, particular attention must be paid to the persistent structural vulnerabilities in the social environment which were again highlighted by the Covid-19 pandemic, informing broader reform and resilience-promoting initiatives and preparations well in advance of the “next one.”

    About the Author

    image of Lawrence Elliott, Associate Professor in the Department of Community Health Sciences at the University of Manitoba.

    Lawrence Elliott is an Associate Professor in the Department of Community Health Sciences at the University of Manitoba. He is a retired public health physician, with training in epidemiology.



    CDC (U.S. Centers for Disease Control) 2011. Principles of Epidemiology in Public Health Practice 3rd ed, CDC, Atlanta GA.

    Jones, E., MacDougall, H., Monnais, L., Hanley, J., Carstairs, C. Beyond the COVID-19 Crisis: Building on Lost Opportunities in the History of Public Health. Royal Society of Canada. 2021.

    Suggested Reading

    Snow, John (1855). On the Mode of Communication of Cholera (2nd ed.). London: John Churchill.