Mr. Daszak told me that, during that second study, the field team took blood samples from a couple of thousand Yunnanese people, about 400 of whom lived near the cave. Roughly 3 percent of them carried antibodies against SARS-related coronaviruses.

“We don’t know if they got sick. We don’t know if they were exposed as children or adults,” Mr. Daszak said. “But what it tells you is that these viruses are making the jump, repeatedly, from bats to humans.” In other words, this Wuhan emergency is no novel event. It’s part of a sequence of related contingencies that stretches back into the past and will stretch forward into the future, as long as current circumstances persist.

So when you’re done worrying about this outbreak, worry about the next one. Or do something about the current circumstances.

Current circumstances include a perilous trade in wildlife for food, with supply chains stretching through Asia, Africa and to a lesser extent, the United States and elsewhere. That trade has now been outlawed in China, on a temporary basis; but it was outlawed also during SARS, then allowed to resume — with bats, civets, porcupines, turtles, bamboo rats, many kinds of birds and other animals piled together in markets such as the one in Wuhan.

Current circumstances also include 7.6 billion hungry humans: some of them impoverished and desperate for protein; some affluent and wasteful and empowered to travel every which way by airplane. These factors are unprecedented on planet Earth: We know from the fossil record, by absence of evidence, that no large-bodied animal has ever been nearly so abundant as humans are now, let alone so effective at arrogating resources. And one consequence of that abundance, that power, and the consequent ecological disturbances is increasing viral exchanges — first from animal to human, then from human to human, sometimes on a pandemic scale.

We invade tropical forests and other wild landscapes, which harbor so many species of animals and plants — and within those creatures, so many unknown viruses. We cut the trees; we kill the animals or cage them and send them to markets. We disrupt ecosystems, and we shake viruses loose from their natural hosts. When that happens, they need a new host. Often, we are it.

The list of such viruses emerging into humans sounds like a grim drumbeat: Machupo, Bolivia, 1961; Marburg, Germany, 1967; Ebola, Zaire and Sudan, 1976; H.I.V., recognized in New York and California, 1981; a form of Hanta (now known as Sin Nombre), southwestern United States, 1993; Hendra, Australia, 1994; bird flu, Hong Kong, 1997; Nipah, Malaysia, 1998; West Nile, New York, 1999; SARS, China, 2002-3; MERS, Saudi Arabia, 2012; Ebola again, West Africa, 2014. And that’s just a selection. Now we have nCoV-2019, the latest thump on the drum.