Interacting contagious diseases like influenza and pneumonia — and perhaps coronavirus too — follow the same complex spreading patterns as social trends, like the adoption of new slang or technologies. This new finding could lead to better tracking and intervention when multiple diseases spread through a population at the same time.
«The interplay of diseases is the norm rather than the exception,» says Laurent Hebert-Dufresne, a complexity scientist at the University of Vermont who co-led the new research. «And yet when we model them, it’s almost always one disease in isolation.»
When disease modelers map an epidemic like coronavirus, Ebola, or the flu, they traditionally treat them as isolated pathogens. Under these so-called «simple» dynamics, it’s generally accepted that the forecasted size of the epidemic will be proportional to the rate of transmission.
But according to Hebert-Dufresne, professor of computer science at University of Vermont, and his co-authors, Samuel Scarpino at Northeastern University, and Jean-Gabriel Young at the University of Michigan, the presence of even one more contagion in the population can dramatically shift the dynamics from simple to complex. Once this shift occurs, microscopic changes in the transmission rate trigger macroscopic jumps in the expected epidemic size — a spreading pattern that social scientists have observed in the adoption of innovative technologies, slang, and other contagious social behaviors.
STAR WARS AND SNEEZING
The researchers first began to compare biological contagions and social contagions in 2015 at the Santa Fe Institute, a transdisciplinary research center where Hebert-Dufresne was modeling how social trends propagate through reinforcement. The classic example of social reinforcement, according to Hebert-Dufresne, is «the phenomenon through which ten friends telling you to go see the new Star Wars movie is different from one friend telling you the same thing ten times.»
Like multiple friends reinforcing a social behavior, the presence of multiple diseases makes an infection more contagious that it would be on its own. Biological diseases can reinforce each other through symptoms, as in the case of a sneezing virus that helps to spread a second infection like pneumonia. Or, one disease can weaken the host’s immune system, making the population more susceptible to a second, third, or additional contagion.
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Materials provided by University of Vermont. Note: Content may be edited for style and length.