Spillover: Animal Infections and the Next Human Pandemic

“Among the people visiting those temples, feeding handouts to those macaques, exposing themselves to SFV, are international tourists. Some carry away more than photos and memories. ‘Viruses have no locomotion,’ according to the eminent virologist Stephen S. Morse, ‘yet many of them have traveled around the world.’ They can’t run, they can’t walk, they can’t swim, they can’t crawl. They ride.” 

Quammen’s catalogue of tourist activities intersperses the mundane with the potentially dangerous: Every activity humans engage in carries the potential for viral exposure. Vacations not explicitly about animal tourism nevertheless produce animal contact. And viruses, for all their limitations, travel. Quammen casts them as passengers, albeit passengers with no independence. This invisible mode of transit, as he establishes in other chapters, has profound implications for everyone an unwitting host meets on the way. 

“These local riddles about Hendra are just small forms of big questions that scientists such as Kate Jones and her team, and the Edinburgh researchers, and Hume Field, and many others around the world are asking. Why do strange new diseases emerge when they do, where they do, as they do, and not elsewhere, other ways, at other times? Is it happening more now than in the past? If so, how are we bringing these afflictions upon ourselves? Can we reverse or mitigate the trends before we’re hit with another devastating pandemic? Can we do that without inflicting fearful punishment on all those other kinds of infected animals with which we share the planet? The dynamics are complicated, the possibilities are many, and while science does its work slowly, we all want a fast response to the biggest question: What sort of nasty bug, with what unforeseen origins and what inexorable impacts, will emerge next?” 

This passage illustrates one of Quammen’s narrative techniques. He poses questions, putting himself in the place of the reader, making it clear that he is another investigator, albeit less experienced than some of the scientists he profiles and interviews. The questions about how and when diseases originate, how responsible humans are, and whether we can stop spillover, linger within every scientific investigation Quammen describes, making Hendra emblematic and a fitting beginning. Scientists ask many of the same questions, though Quammen acknowledges that ordinary readers will likely feel more urgency. Tellingly, he offers no definitive answers, using the conventions of narrative nonfiction to create a compelling atmosphere. 

“It was very hard, he said. He had lost his gorilla family, and also members of his human family. For a long time Prosper stood holding the book, opened for us to see those names. He comprehended emotionally what the scientists who study zoonoses know from their careful observations, their models, their data. People and gorillas, horses and duikers and pigs, monkeys and chimps and bats and viruses: We’re all in this together.” 

Quammen uses the story of the tracker Prosper Balo to further anchor Ebola’s human dimension. Balo grieves for gorillas lost to Ebola. Ebola may seem remote, but Quammen uses this anecdote to illustrate its immediacy: All zoonotic diseases can reach people, and the fate of animal populations is always relevant. Quammen is careful not to dismiss emotional connections: For him, engagement with Ebola on a personal level is also an asset, as it advances his argument about the interconnectedness of life. 

“Apart from the problem (which proved large) of acquired resistance to DDT among Anopheles mosquitoes, the planners and health engineers of WHO probably gave insufficient respect to another consideration—the consideration of small changes and large effects. Humans have an enormous capacity to infect mosquitoes with malaria. Miss one infected person in the surveillance-and-treatment program to eliminate malarial parasites from human hosts, and let that person be bitten by one uninfected mosquito—it all starts again.” 

This episode in the story of humanity’s struggle with malaria highlights the challenges of disease eradication. It only takes one infected person to introduce malaria to a mosquito, and in turn for mosquitos to infect more people. No detection mechanism is perfect, and no surveillance program can track an entire population. Mosquitoes resist humanity’s effort to eradicate them, and humans in turn inadvertently resist being easy to identify as carriers. As is often the case in Quammen’s narrative, humanity is just one more animal spreading disease. 

“‘Have we created this nice opening for knowlesi to come into?’ It was Cox-Singh voicing the question. By ‘opening’ she meant an ecological opportunity. ‘What’s a mosquito going to do? If we start taking so much of the habitat, will the mosquito adapt then to being in a less-forest environment?’ She let that thought trickle off, paused, and then started again. ‘I honestly believe we’re at a sort of critical point. And we should be watching. We should be watching the situation very, very carefully,’ she said. ‘And hopefully nothing will happen.’ But of course, as she well knew, something always does happen. It’s just a question of what and when.” 

While Plasmodium knowlesi is perhaps less fearsome than some of Quammen’s subjects, the questions here neatly illustrate why human behavior is cause for anxiety. Habitat change and deforestation are likely to continue, and it is difficult even for an expert like Cox-Singh to predict how mosquitos will respond to a likely drop in accessibility of monkeys. She prescribes constant vigilance, which Quammen echoes when he notes that her “hope” nothing will happen is largely meaningless, since it cannot dictate either the response of the mosquito or the virus. Again, humans are unwitting, active agents in their own destruction: The “opening” for the virus is the result of ongoing choice, not random, fearsome forces beyond our control.

“The very name coined during that early period, SARS, reflects the fact that this thing was known only by its effects, its impacts, like the footprints of a large, invisible beast. Ebola is a virus. Hendra is a virus. Nipah is a virus. SARS is a syndrome.” 

The description of SARS here is particularly fearsome, establishing it as a mystery that invites anxiety and chills the reader. What we call things matters, to Quammen: He defines all scientific terms carefully and lucidly. He notices here that SARS has retained an air of mystery long after the initial outbreak. While Quammen emphasizes that viruses are not human, and do not have motives, in this passage he invites the reader to experience the fear and uncertainty that plagued much of the world in 2003. Quammen uses narrative techniques to engage the reader with a complicated subject, not shying away from emotion when it helps to elucidate why a particular outbreak matters. 

“He pointed to a biohazardous waste box, specially designed for accepting suspect materials. ‘But if they were food,’ he added, ‘they’d go there,’ indicating an ordinary trash basket against the wall. It was a shrug back toward our dinner discussions and the tangled matter of categorical lines: edible animals versus sacrosanct animals, safe animals versus infected animals, dangerous offal versus garbage. His point again was that such lines of division, especially in southern China, are arbitrarily and imperfectly drawn.” 

Quammen’s work with Aleksei Chmura reminds him how fluid the boundaries are that humans draw between food, scientific specimens, and the “wild.” Bats that do not survive their encounters with the laboratory are designated as hazardous, while the same animals are discarded in ordinary trash after a meal. Safety is a shifting, amorphous concept, like the boundary between a meal and a laboratory animal: Toward the end of his journey, Quammen begins to accept that it may even be an illusion. Human categories may be useful, even necessary, but they are no guarantee of security and may not truly hold up to scrutiny. 

“Then I boarded my onward flight. In the row beside me were two young Japanese tourists, a couple, possibly returning from a romantic vacation amid the hotels, parks, malls, markets, restaurants, and crowded streets of Guangzhou or other cities of southern China. They took their seats unobtrusively and settled in for the short ride to Hong Kong. Maybe they felt a bit cowed by their own adventurousness and relieved to be headed home to a tidier nation; maybe they remembered the news stories about SARS. I didn’t intrude on them with questions. I wouldn’t have noticed them at all, except they were both wearing surgical masks. Yes, I thought, if only it were that simple.” 

Here, Quammen applies his role as observer to ordinary people. It is noteworthy that after many pages recounting the role of travel in spreading SARS, Quammen himself boars a plane to return home. His knowledge does not prevent him from engaging in a possibly dangerous activity; some of the activities that spread zoonotic disease are indispensable to modern life. Quammen speculates an entire life for these people, engaging briefly in more speculative narration, closer to fiction than reality. He is brought back to his principle concern by the sight of their surgical masks. He, and his readers, will never know whether those masks were sufficient for the Japanese tourists: we can only accept that human precautions can fail and continue to go about daily life anyway. 

“The rate of new human cases declined from its 2009 peak. By the middle of July 2010, only 420 more Netherlanders had been diagnosed with Q fever. The ministry officials could feel guardedly optimistic that their public health crisis had been brought under control. The doctors could relax slightly. The dairy farmers could lament their losses. But the scientists knew that Coxiella burnetii wasn’t gone. It had waited for ideal conditions before, and it could wait again.” 

This summation of the end of the Q fever outbreak brings home that large-scale outbreak of zoonotic disease is not simply a matter for poor countries with less technological or medical sophistication. It took the Netherlands several years to successfully contain its outbreak. But eradication is not a realistic goal, and scientists remain watchful sentries even as ordinary people begin to relax and move on. Here, Quammen personifies the bacterium, noting that another outbreak may just be a matter of time: Where humans rush to contain outbreaks, the disease can wait patiently for more opportunities.

“So forget about deer abundance. White-tailed deer are involved in the Lyme disease system, yes, but involved like a trace element, a catalyst. Their presence is important but their numerousness is not. The littler mammals are far more critical in determining the scale of disease risk to people. Adventitious years of big acorn crops, yielding population explosions of mice and chipmunks, are more likely to influence the number of Lyme disease cases among Connecticut children than anything that deer hunters may do. Beyond helping the blacklegged tick (infected or uninfected) to survive, white-tailed deer are almost irrelevant to Lyme disease epidemiology.” 

This summation of the more complex ecology of Lyme disease sums up the danger of looking at the most obvious phenomenon first. Deer are not insignificant, but they obscure a more complex ecological reality. Human activity is, in this instance, somewhat less relevant to containing disease: Younger ticks rely on smaller animals, and younger ticks spread Lyme. The real role for humanity here, Quammen posits, is to be thoughtful and aware—and, perhaps, to support biodiversity in woodlands. 

“Nobody is claiming, not so far, anyway, that Lyme disease can likewise travel on the wind. Both the round bodies of B. burgdorferi and the small form of C. burnetii merely illustrate that, even in the age of antibiotics, bacteria can be sneaky and tough. These microbes remind us that you don’t have to be a virus to cause severe, intractable, mystifying outbreaks of zoonotic disease in the twenty-first century. Although it helps.” 

Quammen uses his narratives of bacterial disease to craft a more multifaceted narrative of zoonosis. This effort is comparable to that of disease ecologists: Just as they wanted to stop people and local governments from blaming white-tailed deer for Lyme disease, he ensures that his readers will not solely blame viruses for complex outbreaks. Quammen calls bacteria “sneaky” and “tough,” once again personifying disease for the reader, even though he urges rational approaches in most other instances. The word choice here emphasizes that bacteria remain resilient even as human technology has advanced. 

“And like all forms of ecological equilibrium, it’s temporary, provisional, contingent. When spillover occurs, sending a virus into a new kind of host, the truce is canceled. The tolerance is nontransferable. The equilibrium is ruptured. An entirely new relationship occurs. Freshly established in an unfamiliar host, the virus may prove to be an innocuous passenger, or a moderate nuisance, or a scourge. It all depends.” 

Quammen emphasizes here that for viruses, stability is a rare phenomenon. Reservoir host situations are common, but they are also not guaranteed. Spillover is an irreversible change, marking an entirely new reality. This is compared to the end of a truce: Viruses are martial, ready to battle with their new hosts. However, the new circumstances are not certain: There is an entire universe of possibility, and only some of those possibilities are truly terrifying. Every spillover is a risk, and the implications only become clear with infection—and time.

“‘Herpes B gets populations of monkeys shot in the head and…’—she had in mind the safari park culling as well as other such events—‘just eradicated. Herpes B is like Ebola that way.’ It’s not only frightful and potent, she meant, but profoundly misunderstood. Herpes B and Ebola, of course, are very different sorts of bug. But she was right; there are similarities worth noting. In both cases, the virus is often lethal to humans but not nearly so consequential as it would be if not constrained by the limits of its transmissibility. It has no preternatural powers. It finds humans a dead-end host. People are ignorant about its actual properties and inclined to imagine an unreal breadth of risk.”

Quoting anthropologist Lisa Jones-Engel, Quammen stresses that human response to disease is often irrational and unhelpful. Jones-Engel is concerned that monkeys die needlessly as a response to human ignorance. In comparing herpes B to Ebola, Quammen endeavors to point out that viruses resemble each other and not humans: There is no malevolent intelligence at work, and each virus currently has inherent limits to harming humans. Implicitly, then, the value of expertise is to put fear and anxiety in proper perspective. 

“Who makes these rules? Unless you’re a creationist, you’ll likely recognize that the answer is nobody. Where do they come from? Evolution. They are life-history strategies, carved by evolutionary chisels from a broader universe of possibilities. They persist because they work. You can find it in Darwin: descent with modification, natural selection, adaptation. The only surprise, if it is a surprise, is that viruses evolve just as surely as creatures that are unambiguously alive.” 

For Quammen, the rules of viral behavior are distinct from a moral code or the rules of etiquette. Instead, natural laws determine which viruses thrive and which do not. They persist not out of some effort of will, but because success leads to additional success. Further, these mysteries are not new, as Darwin’s work was published in the 19th century. This gap in popular knowledge is due to ignorance, not lack of expert knowledge. It may be somewhat unexpected that creatures that lack the technical attributes of life nevertheless evolve and respond to conditions, but it is not inexplicable. 

“Forget about palm civets, for a moment, and consider mass-production animal husbandry. It’s almost impossible to screen your pigs, cows, chickens, ducks, sheep, and goats for a virus of any sort until you’ve identified that virus (or at least a close relative), and we have only begun trying. The larger meaning of Nipah, in accord with Hume Field’s ‘intriguing thought,’ is that tomorrow’s pandemic zoonosis may be no more than ‘a blip on the productivity output’ of some livestock industry today.”

While the spread of SARS was made terrifying by air travel, Nipah is concerning because it is linked to an essentially unavoidable activity: eating. The first rule of zoonosis is that animals carry diseases. Hume Field returns to the narrative to remind Quammen, and the reader, that we do not have anything like a complete inventory of what those diseases are and how they might interact with human immune systems. The only thing we do know is that time is on the side of viruses, rather than humanity, because they can always evolve to become dangerous before experts even know they are there. 

“Rates of replication and mutation of an RNA virus, differential success for different strains of the virus, adaptation of the virus to a new host—that’s evolution. It happens within a population of some organism, as the population responds to its environment over time. Among the most important things to remember about evolution—and about its primary mechanism, natural selection, as limned by Darwin and his successors—is that it doesn’t have purposes. It only has results. To believe otherwise is to embrace a teleological fallacy that carries emotive appeal (‘the revenge of the rain forest’) but misleads. This is what Jon Epstein was getting at. Don’t imagine that these viruses have a deliberate strategy, he said. Don’t think that they bear some malign onus against humans. ‘It’s all about opportunity.’ They don’t come after us. In one way or another, we go to them.” 

While he uses emotional language about disease at many points in his narrative, here Quammen makes a case for objectivity. There is no logic to natural selection—to viral evolution—beyond attempts at survival and propagation. The search for an intelligence leads nowhere. The intelligence here is our own: Humans move into new environments and encounter viruses that were already present. Our intent may not be malign, but its results, as Quammen routinely demonstrates, can be disastrous. 

“It worries the flu scientists because they know that H5N1 influenza is (1) extremely virulent in people, with a high lethality though a relatively low number of cases, and yet (2) poorly transmissible, so far, from human to human. It’ll kill you if you catch it, very likely, but you’re unlikely to catch it except by butchering an infected chicken. Most of us don’t butcher our own chickens, and health officials all over the world have been working hard to assure that the chickens we handle—dead, disarticulated, wrapped in plastic or otherwise—have not been infected. But if H5N1 mutates or reassembles itself in just the right way, if it adapts for human-to-human transmission, then H5N1 could become the biggest and fastest killer disease since 1918.” 

This summation of expert knowledge brings home the contrast between popular anxiety and scientific anxiety. Bird influenza has one attribute that is already worrisome: It kills humans with ease. It currently lacks the ability to spread easily, but this does not put scientists at ease, because they know about viral evolution. Once transmissibility matches virulence, H5N1 becomes a crisis. The imagery here is routine—chickens packaged in plastic are a familiar sight in grocery stores—but the potential inside this daily habit is far from ordinary. 

“After my detour through the Cholera Hospital, after being pinioned by that mother’s expectant stare, I found myself asking the same thing: Why obsess about zoonoses? In the larger balance of miseries, what makes anyone think they should be taken so seriously? It’s a fair question but there are good answers. Some of those answers are intricate and speculative. Some are subjective. Others are objective and blunt. The bluntest is this: AIDS.” 

In Bangladesh, Quammen confronts the moral dilemmas of his work. Even as he investigates the outbreak of relatively rare diseases, he is routinely surrounded by other forms of misery and suffering, much of it related to the spread of disease and lack of health care. While he turns to experts to explain zoonosis, Quammen has his own rapid answer for why it still matters even as Bangladeshi mothers lack medical treatment. AIDS is zoonotic, and its spread has had global consequences and caused profound suffering, as Quammen goes on to explain. 

“Instead they represent midpoints in the course of the pandemic, marking the stage at which a slowly building, almost unnoticeable phenomenon suddenly rose to a crescendo. Again in the dry terms of the disease mathematicians, whose work is vitally applicable to the story of AIDS: R0 for the virus in question had exceeded 1.0, by some margin, and the plague was on. But the real beginning of AIDS lay elsewhere, and more decades passed while a few scientists worked to discover it.” 

Quammen tells the stories of some of the first documented AIDS patients both to establish a human story and also to advance his broader argument. As in the case of Lyme disease, AIDS demonstrates that what we first observe in a disease is not the origin point. These people prove the mathematics of disease is correct, and they elicit sympathy and concern, but they are not the origin. That mystery would only be solved later. It takes more expertise, more time, and more cases to truly unpack the entire story. 

“In other words, HIV hasn’t happened to humanity just once. It has happened at least a dozen times—a dozen that we know of, and probably many more times in earlier history. Therefore it wasn’t a highly improbable event. It wasn’t a singular piece of vastly unlikely bad luck, striking humankind with devastating results—like a comet come knuckleballing across the infinitude of space to smack planet Earth and extinguish the dinosaurs. No. The arrival of HIV in human bloodstreams was, on the contrary, part of a small trend. Due to the nature of our interactions with African primates, it seems to occur pretty often.” 

Quammen sums up the various groups of AIDS in simple terms: They are very similar viruses, but distinct enough from one another to each be a single spillover. This means that HIV, for all its power to frighten, is in some sense commonplace, the price of routine human engagement with African primates. It is not a one-time crisis but ongoing—there is no moral value to assign, or lesson on the nature of randomness. Instead, it demonstrates Quammen’s ongoing theme that humans always encounter animals and are intimately connected to them.

“During just the year 1937, throughout French Equatorial Africa, the army of doctors and nurses and semipro jabbers delivered 588,086 injections aimed at trypanosomiasis, not to mention countless more for other diseases. Pepin’s arithmetic totaled up 3.9 million injections just against trypanosomiasis, of which 74 percent were intravenous (right into a vein, not just a muscle), the most direct method of drug delivery and also the best for unintentionally transmitting a blood-borne virus. All those injections, according to Pepin, might account for boosting the incidence of HIV infection beyond a critical threshold. Once the reusable needles and syringes put the virus into enough people—say, several hundred—it wouldn’t come to a dead end, it wouldn’t burn out, and sexual transmission could do the rest.” 

This description of infectious disease campaigns highlights both the power of unintended consequences and humanity’s role in exacerbating them. Quammen characterizes the public health workers conducting various infectious campaigns as an “army”: While they thought of themselves as fighting disease, they inadvertently joined the ranks of another they did not know about and exacerbated the spread of HIV. Like SARS superspreaders, health professionals may play disproportionate roles in helping zoonotic disease thrive. 

“For a dozen years it traveled quietly from person to person. Symptoms were slow to arise. Death lagged some distance behind. No one knew. This virus was patient, unlike Ebola, unlike Marburg. More patient even than rabies, but equally lethal. Somebody gave it to Gaëtan Dugas. Somebody gave it to Randy Shilts. Somebody gave it to a thirty-three-year-old Los Angeles man, who eventually fell ill with pneumonia and a weird oral fungus and, in March 1981, walked into the office of Dr. Michael Gottlieb.” 

The conclusion of Quammen’s HIV/AIDs narrative has parallels to his earlier case studies, while further demonstrating his use of narrative technique. HIV is “patient,” again imbued with human characteristics, but Quammen compares it not to other people but to other diseases he has studied before. HIV sheds its anonymity and enters patients Quammen has previously identified. Like Hendra, HIV traveled—not on its own, but in the bodies of its host, eventually encountering the medical experts who would seek to identify it. 

“A trillion pounds of cows, fattening in feedlots and grazing on landscapes that formerly supported wild herbivores, are just another form of human impact. They’re a proxy measure of our appetites, and we are hungry. We are prodigious, we are unprecedented. We are phenomenal. No other primate has ever weighed upon the planet to anything like this degree. In ecological terms, we are almost paradoxical: large-bodied and long-lived but grotesquely abundant. We are an outbreak.” 

This excerpt from Quammen’s long description of humanity’s outbreak potential and expanded presence is staggering in its scale. In focusing on the weight of the cows rather than their number, Quammen leads the reader to imagine the earth itself as literally weighed down by the force of our food supply. As in the Nipah chapters, the presence of livestock is a measure of our impact and our potential: When Quammen emphasizes later that all outbreaks end, it is easy to imagine, given his past narratives of zoonosis, that cows or pigs could be part of that process. From an ecological perspective, our intelligence matters less than our size and lifespan—and we have reached outbreak potential, just like Quammen’s caterpillars.