Spillover: Animal Infections and the Next Human Pandemic

These are safety designations for medical research laboratories, referring to the “biosafety level” of each. BSL-4, pioneered by scientist Karl Johnson, involves the most elaborate containment procedures, which Quammen describes as “multiple seals, negative air pressure, elaborate filters, and lab personnel working in space suits—a containment zone in which Ebola virus could be handled without risk (74). BSL-3 laboratories are for diseases that are not only harmful and highly contagious but also treatable, like plague. 

This situation describes the status of a virus at the end of an outbreak, from the perspective of the virus. If a spillover is unsuccessful—leading to relatively few new infections and ultimately ending the number of cases—that host is a dead-end host. This describes only that particular case, as Quammen highlights: “Not the virus in toto throughout its range, of course, but that lineage of virus, the one that has spilled over, betting everything on this gambit—it’s gone, caput” (83). For Ebola, humans are routinely a dead-end host. This state of affairs may well be temporary, as viruses often evolve and can thus turn dead-end hosts into opportunities for emergence. 

This is what happens when spillovers recur enough for a virus to sustain itself. Quammen cites the definition that an emerging disease is “an infectious disease whose incidence is increasing following its first introduction into a new host population” (42). AIDS is Quammen’s main example of successful emergence. Where spillovers are single events, “emergence is a process, a trend” (43). Some of the diseases Quammen studies have spilled over into humans, but this has not led to sustained emergence. 

The branch of biology dedicated to studying the relationships, historical and current, between different viruses:

A molecular phylogeneticist scrutinizes the nucleotide sequences in the DNA or RNA of different organisms, comparing and contrasting, for the same reason a paleontologist scrutinizes fragments of petrified bone from extinct giant saurians—to learn the shape of lineages and the story of evolutionary descent (422).

This is the subgroup of virology that helps scientists identify exactly how and when HIV spilled into humans—and how simian and human immunodeficiency viruses differ from one another. It is one means of establishing the reservoir host of a virus, as the close relationship between chimp SIV and human HIV proves that chimps were the reservoir for the virus in the early 20th century. 

This term refers not only to diseases, but also to population ecology. As Quammen notes, quoting an eminent entomologist: “from the ecological point of view an outbreak can be defined as an explosive increase in the abundance of a particular species that occurs over a relatively short period of time” (496). Humanity’s rapid growth and spread into new habitats is the most significant recent outbreak on earth. It also explains why spillovers are frequent and increasing in recent decades: humans are routinely disrupting habitats, encountering animals in new ways, and changing the dynamics of animal populations. Most of Quammen’s subjects are familiar with a key aspect of outbreaks: They are not permanent. This is part of why scientists are so preoccupied with the nature of the “Next Big One” (42).

The colloquial term for R0, the “basis reproduction rate” of a given infection. The term was established by virologist George Macdonald in the 1950s to refer to “the number of infections distributed in a community as the direct result of the presence in it of a single primary non-immune case” (146). That number must be greater than one for a disease to persist, and when it is significantly greater than that, an outbreak is much larger and potentially disastrous. R-naught for HIV eventually rose to a high enough level to cause a global pandemic. 

This is the term given to an animal that routinely carries a virus or bacteria but does not become ill from this relationship. Usually this balance is achieved over millions of years. The search for reservoir hosts is key to containing outbreaks, as it is the only reliable way to stop spillovers and emergence. These reservoir hunts are often the main subject of Quammen’s chapters and a principle preoccupation of many of his protagonists. 

The genetic material of a virus has a direct relationship to how quickly it mutates—and thus how easily it might turn a spillover into an opportunity for an epidemic. RNA lacks DNA’s double helix and capacity to repair itself—it compensates for this, however, because “RNA viruses mutate profligately” (308). They also “jump species a lot” (309) to compensate for their smaller genomes. This difference between RNA viruses and DNA ones also goes some way to explaining why the former are more commonly seen in studies and narratives of zoonotic emerging diseases. 

This model for outbreaks came from Scottish mathematicians Kermack and MacKendrick, who sought to explain the roles of different groups in a population during the introduction of a new disease. S refers to “susceptible,” I stands for “infected,” and R stands for “recovered” (143). There is a mathematical relationship between these groups, such that “the number of recovered individuals in the population, at a given moment, reflects the number of infected individuals times the average recovery rate” (143). The formula is used routinely by scientists to describe the state of a population dealing with a particular disease. 

This term recurs so often it is also Quammen’s choice of title. It is “the term used by disease ecologists (it has a different use for economists) to denote the moment when a pathogen passes from members of one species, as host, into members of another” (43). Spillover is a single event, involving the infection of one animal. Quammen’s first example of this is in his Hendra chapter when the horse is infected. The horse’s trainer becoming infected is a second spillover. 

Quammen introduces this term in his SARS section to refer to any “patient who, for one reason or another, directly infects far more people than does the typical infected patient” (172). Superspreaders were particularly influential in the 2003 SARS outbreak. They illustrate that while spillover is a result of a human-animal encounter, zoonotic diseases spread because of human choices and unknown factors.

The means and ease with which a virus travels “from one host to another” (291). Viruses that can travel airborne, like SARS, are highly transmissible. “Ordinary transmission” is the means by which a disease travels from host to host in its reservoir; other cases of transmission in a newer host are classified as “extraordinary” (293). 

Loosely stated, the “measurable degree of […] disease” that a particular virus contains (294). Inside of their reservoir hosts, many viruses are not especially virulent, having achieved relative balance between the needs of the two organisms. High virulence is not necessarily bad for a virus, as long as it manages to achieve transmission before the host dies.