the level of 7 billion and beyond. We are well on our way toward 9 billion before our growth trend is likely to flatten. We live at high densities in many cities. We have penetrated, and we continue to penetrate, the last great forests and other wild ecosystems of the planet, disrupting the physical structures and the ecological communities of such places. We cut our way through the Congo. We cut our way through the Amazon. We cut our way through Borneo. We cut our way through Madagascar. We cut our way through New Guinea and northeastern Australia. We shake the trees, figuratively and literally, and things fall out. We kill and butcher and eat many of the wild animals found there. We settle in those places, creating villages,
work camps, towns, extractive industries, new cities. We bring in our domesticated animals, replacing the wild herbivores with livestock. We multiply our livestock as we’ve multiplied ourselves, operating huge factory-scale operations involving thousands of cattle, pigs, chickens, ducks, sheep, and goats, not to mention hundreds of bamboo rats and palm civets, all confined en masse within pens and corrals, under conditions that allow those domestics and semidomestics to acquire infectious pathogens from external sources (such as bats roosting over the pig pens), to share those infections with one another, and to provide abundant opportunities for the pathogens to evolve new forms, some of which are capable of infecting a human as well as a cow or a duck. We treat many of those stock animals with prophylactic doses of antibiotics and other drugs, intended not to cure them but to foster their weight gain and maintain their health just sufficiently for profitable sale and slaughter, and in doing that we encourage the evolution of resistant bacteria. We export and import livestock across great distances and at high speeds. We export and import other live animals, especially primates, for medical research. We export and import wild animals as exotic pets. We export and import animal skins, contraband bushmeat, and plants, some of which carry secret microbial passengers. We travel, moving between cities and continents even more quickly than our transported livestock. We stay in hotels where strangers sneeze and vomit. We eat in restaurants where the cook may have butchered a porcupine before working on our scallops. We visit monkey temples in Asia, live markets in India, picturesque villages in South America, dusty archeological sites in New Mexico, dairy towns in the Netherlands, bat caves in East Africa, racetracks in Australia—breathing the air, feeding the animals, touching things, shaking hands with the friendly locals—and then we jump on our planes and fly home. We get bitten by mosquitoes and ticks. We alter the global climate with our carbon emissions, which may in turn alter the latitudinal ranges within which those mosquitoes and ticks live. We provide an irresistible opportunity for enterprising microbes by the ubiquity and abundance of our human bodies.
Everything I’ve just mentioned is encompassed within this rubric: the ecology and evolutionary biology of zoonotic diseases. Ecological circumstance provides opportunity for spillover. Evolution seizes opportunity, explores possibilities, and helps convert spillovers to pandemics.
It’s a neat but sterile historical coincidence that the germ theories of disease came to scientific prominence at about the same time, in the late nineteenth century, as the Darwinian theory of evolution—neat because these were two great bodies of insight with much to offer each other, and sterile because their synergy was long delayed, as germ theories remained for another sixty years largely uninformed by evolutionary thinking. Ecological thinking, in its modern form, arose even later and was equally slow to be absorbed by disease science. The other absent science, until the second half of the twentieth century, was molecular biology. Medical people of the earlier eras might guess that bubonic plague was somehow related to rodents, yes, but they didn’t know how or why until Alexandre Yersin, during an 1894 epidemic in Hong Kong, found the plague bacterium in rats. Even that didn’t illuminate the path to human infection until Paul-Louis Simond, several years later, showed that the bacterium is transmitted by rat fleas. Anthrax, caused by another bacterium, was known to kill cows and people but seemed to arise by spontaneous generation until Koch proved otherwise in 1876. Rabies was even more obviously associated with transmission to humans from animals—notably, mad dogs—and Pasteur introduced a rabies vaccine in 1885, injecting a bitten boy, who survived. But rabies virus itself, so much smaller than a bacterium, couldn’t be