“It is interesting to note that this epidemic displayed one of the characteristics so often referred to in modern epidemiology — namely, when the outbreaks begin, the number of sick and the mortality were relatively slight, but both rose with appalling violence as the epidemic gathered velocity.”
It is understood that while worsening conditions of life in an epidemic exact a toll, the primary change is increase in the virulence of the pathogen.
Zinsser had personal experience with the notorious modern example. (NCBI PMC) The U.S. Military and the Influenza Pandemic of 1918–1919 relates the observations of medical officer Alan M. Chesney. Quoting (boldface mine),
During Chesney’s first documented period, the month of June to July 27, the 5th Artillery Brigade had 77 “relatively mild” cases of influenza. During the second phase, July 27 to August 23, 200 men of the 58th Artillery Brigade became ill, about 6.5%. None of them died, but the outbreak was serious enough that the next brigade cleaned out the barracks, even washing the walls, before they moved in. Despite this precaution, during Chesney’s third phase, August 23 to November 8, more than one-third of the 6th Artillery Brigade, 1,636 soldiers, contracted influenza and 151 died. Chesney concluded that “…these successive outbreaks tended to be progressively more severe both in character and extent, which would speak for an increasing virulence of the causative agent.”13
Like the stories of Zinsser’s captivating book, this factual account omits the mechanics of why virulence can increase. Since that time, epidemiology has evolved from a preponderance of mystery to an inexact science. In the past several months, epidemiologists have used a lot of hopeful words, with reluctance to make predictions. Only within the past week or so has CDC confided that COVID-19 is inescapable.
Like the social sciences, epidemiology lacks the bedrock of hard science master equations. But there is math; it just has to be tweaked to fit the circumstances. The next article offers an intuitive approach to Zinsser’s assertion and Chesney’s experience. The predator-prey equations, explained in words, are a good place to start. Intuitive epidemiology is accessible to a large audience., including you.
The warning: In large parts of the world, particularly within conflict zones, but also without, conditions exist for an increase in the virulence of COVID-19.
To be continued shortly.