It Is What It Is
Before one even begins a discussion on infectious disease and public health, I think it is important to define the pertinent terms based upon generally accepted epidemiological principles. When I refer to an epidemic, I’m referring to the rapid spread of a disease to a large number of people in a given population within a short period of time. The disease may or may not be normally expected in the population in that area. An outbreak is also a rapid spread of disease to a large number of people but is frequently used when referring to a more limited geographic area than that used for an epidemic. A cluster is an aggregation of cases grouped in place and time that are suspected to be greater than the number expected, even if the expected number is not precisely known. A pandemic is an epidemic that has spread over multiple countries, regions or continents. It can be thought of as a traveling epidemic—add a P to epidemic for a passport {smile}. In fact, a useful way of differentiating these terms is to think in terms of the spread of a communicable agent: A cluster is initially observed, then it becomes an outbreak, which then becomes an epidemic which then becomes a pandemic.
Sporadic refers to a disease that occurs infrequently and irregularly, such as tetanus or rabies. Endemic refers to the constant presence of an infectious agent in a population within a geographic area, such as chickenpox in the United States or Hepatitis B worldwide. Hyperendemic refers to persistent, high levels of disease occurrence such as malaria in Africa or dengue in Brazil or Papua New Guinea. An interesting observation is that diseases such as tetanus or rabies have never been known to reach epidemic status and therefore remain sporadic, whereas communicable agents that have reached epidemic or pandemic levels frequently then become endemic to human population. Seasonal influenza and small pox are examples of the latter and fortunately can be ameliorated with vaccine.
Covid-19 has killed 700,000 humans globally, 160,000 Americans. It was classified officially as a pandemic in 2019, becoming the approximate 17th pandemic in known human history. The Influenza pandemic of 1918 is frequently referenced for comparison purposes to the current Covid-19 pandemic. And, although actual numbers are subject to all the caveats inherent in data collection, the number of deaths attributed to the 1918 influenza pandemic (approximately 20-50 million worldwide) may in fact be surpassed by the on-going HIV/AIDS pandemic which has killed approximately 40 million humans and continues to kill at a rate of approximately 1-2 million annually.
In the past 2500 years, humanity has experienced 250 epidemics, including pandemics, that have killed billions of human beings. I am referring to diseases caused by a communicable agent, such as a virus or a bacteria, and not to the presence of widespread, non-communicable diseases such as cancer or heart disease that also take a mortality bite on the human life-span. The transmissible agents most responsible for the highest death toll have been Yersinia Pestis, Variola major and Variola minor, Influenza A and B, and Measles, causing Bubonic Plague, Smallpox, Influenza, and Measles respectively.
Without a doubt, the Bubonic Plague ranks as one of the greatest killers of all time and constitutes 55 of the known 250 epidemics. The earliest known instance was the Plague of Justinian, a year long reign of terror in the 6th century which is thought to have killed half the population of Europe and, at its height, killed an estimated 5000 people per day in the city of Constantinople. Since that time, Yersinia Pestis continues to murder human beings even today with hundreds dying from it in Madagascar in 2014.
Smallpox, possibly the etiologic agent for the Antonine Plague that swept through the Roman Empire in 165 AD, accounts for 25 of the 250 epidemics with a death toll of approximately 16 million. Smallpox is an outstanding example of humans working together to eradicate a disease, successfully achieving that goal in 1980. The last known case of smallpox occurred in an outbreak in the United Kingdom in 1978 when Janet Parker, a medical photographer, contracted the disease and died. The source of her infection was traced to the smallpox virus grown for research purposes at the Medical School laboratory. Although the World Health Organization recommended destruction of the virus in 1986, resistance from the United States and Russia prompted instead temporary retention of virus stocks for specific research purposes. It is another, very long discussion on whether the continued presence of viable virus serves an essential public health purpose or a possible threat to humanity.
Influenza accounts for 20 of the 250 known epidemics in human history with the 1918 pandemic causing approximately 20-50 million global deaths. The death toll, however, like HIV/AIDS, continues to rise every year with seasonal flu deaths estimated at 60,000 annually.
The other frequently occurring pathogens implicated in our history of epidemics include cholera, measles, typhoid fever, yellow fever, malaria among other pathogens that have occurred less frequently but still with devastating fatality rates, such as Ebola. The Zika, SARS, MERS epidemics are notorious for being new kids on the block of human epidemics. An etiologic agent that I am watching closely (as a retired member of the medical community so strictly as an interested observer) is the increasing frequency of epidemics attributed to dengue fever—16 epidemics in a variety of countries in the last 20 years.
It should be noted that in the last 10 years alone humanity has seen 20 epidemics occurring all over the world and involving the old standbys, such as cholera, measles, and influenza, but less well known agents such as Nipah, Japanese Encephalitis, and Chikungunya. This year alone, measles killed over 8000 souls in the Democratic Republic of the Congo, New Zealand, the Philippines, and Samoa while Dengue fever has killed approximately 4000 in Asia-Pacific and Latin America. Those epidemics should be included with the on-going Covid-19 pandemic with a death toll of 700,000 and counting.
I can only say to those who think of the Covid-19 pandemic as a ‘scourge’ that came out of ‘nowhere’ and as ‘something the world has not seen for a long, long time’—You. Haven’t. Been. Paying. Attention!
Diana Lyn Davidson, August 2019
