We’re in an outbreak currently. But I don’t mean the COVID pandemic. It turns out that outbreak has a second definition: it means when populations grow dramatically large, beyond their carrying capacities. As David Quammen details in his book Spillover, disease outbreaks can be considered “as a subset” in this broader category. 
In Spillover, Quammen goes into detail about one outbreak—that of tent caterpillars—and how, inevitably, it is controlled by a virus that literally melts them from the inside. These viruses are then eaten by other caterpillars as they chomp away, dissolving them over time, and spreading more virons on the leaves future caterpillars will eat. Eventually, in a dense outbreak of caterpillars, the virus wipes out much, if not all of the population.
Quammen makes the case that humans, too, are in outbreak (not that a case really needs to be made), citing: our huge numbers (8 billion and heading to 10 billion*); our dense cities; our mobility; our continuing expansion into forests and wild ecosystems; our large and dense populations of livestock; our misuse of antibiotics, especially on said livestock populations; and climate change (spreading disease vectors’ ranges). And he says it with pizazz: “We provide an irresistible opportunity for enterprising microbes by the ubiquity and abundance of our human bodies.” (That feels like it should be on a corporate recruiting poster. “Ask about our signing bonus!”) 
But he also makes the essential point that humans cannot be separated from the natural world—or more accurately, he notes, “There is no ‘natural world’….There is only the world. Humankind is part of that world, as are ebolaviruses, as are influenzas and the HIVs…, as are chimpanzees and bats…, as is the next murderous virus—the one we haven’t detected yet.” (Did I mention Quammen wrote Spillover in 2012?) 
That’s a key point. Viruses are part of the Gaian whole. Indeed, they are a primary means of stabilizing overgrown populations. This is not a mystical statement, or some suggestion that Gaia has sent us a plague. As Quammen notes, “evolution seizes opportunity.” When there’s a big, connected, genetically similar population, viruses, bacteria, and other disease vectors view this as an all-you-can-eat buffet, if they can just figure out how to get into the restaurant.
Human Behaviors and the Spread of Viruses
And in our case, humans have pretty much thrown open the door. As Quammen points out several times, the effects of a pandemic depend on whether we act ‘diligently or doltishly,’ and concludes the book with one epidemiologist (the tent caterpillar expert) saying that unlike caterpillars, we’re smart. We have a lot of ‘heterogeneity’ [variety] in our behavior and we can make many choices to limit infection and impede its spread. Quammen even includes a long list of behavioral modifications from not sharing needles and avoiding unprotected sex to not eating bushmeat and not penning pigs beneath mango trees. 
Spillover, of course, was written before COVID, and we are proving again and again that we are not as smart as we could be. In fact, we’re doing deeply stupid things—like not wearing masks. Or traveling for Thanksgiving. Or going to gyms and restaurants. I cannot process this with a rational mind. Instead I wonder, whether, as with certain species of grasshoppers, do outbreak dynamics lead us to behave differently?