Contagion rates: How they are affected by knowledge and behavior
“How contagious is the disease?” is an important question.
The rate of contagion is the expected number of new cases of a disease that is likely to be caused by one case in a population where all persons in the population are susceptible. This is not necessarily how dangerous a disease is: one disease may be 100 percent fatal, but not contagious, another may spread like wildfire, but with little to no symptoms.
This rate of contagion is called the basic reproductive number R0 (or R-zero). This number also depends on the health and conditions of other people in the population. Some individuals may be immune due to prior exposure or vaccination. Others may have behaviors (e.g. “social distancing”) that do not bring them into as much contact with infected carriers.
Therefore the rate of contagion varies among disease agents, and is a calculation of the average number of new cases that will arise from exposure to one infected person. This R0 varies for the following agents.
But the influenza epidemic of 1918 that began in Kansas and spread worldwide had a rate of only 2–3 but killed an estimated 675,000 in the United States and 50 million worldwide. So this R0 measure alone does not predict how fast the infection spreads or how fatal it will be.
There are other important questions medical researchers are working on answering.
To what extent are there silent carriers who show no symptoms but who spread the infection?
How long is an affected patient infectious?
Does the disease agent travel as an airborne virus, in airborne droplets or by other contact?
How long can the infectious agent survive in the environment?
And what is the fatality rate if you do contract the disease?
The number of casualties that COVID-19 will cause will also depend on many cultural and environmental factors.
The willingness (or ability) of a population to “stand in place” to limit transfer.
Willingness (or ability) to thoroughly wash hands.
Wearing masks to prevent transmission.
The availability of medical care for severe cases.
Not all of the above factors are entirely under the average citizen’s control. However, understanding of these many factors, as well as of anatomy and physiology and the microbiology of bacteria and viruses allows each of us to take some ownership of the public actions necessary during an epidemic.
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