If you were perusing the library shelves and came across this book, the title “Spillover” may leave you a little uncertain in terms of its topic, without reading the subtitle and having some understanding of the processes described in the book, the emergence, evolution and ecology of ‘new’ diseases, human diseases. When we speak of them we understand them as conditions, assaults on our health and human bodies, which result in an array of symptoms, with wide ranging severity, ranging from mild and asymptomatic; to bothersome with knowable, short lived cycles; through chronic and debilitating; to those entailing a series of feverish cycles we simply must endure; to those often painful and fatal which wreck havoc on our systems and organs. Quammen, the author, is as always, an intelligent and thorough researcher able to interpret complex topics for the layman while staying true to the science and the people whose stories he tells as he weaves together the larger narrative.

Quammen spent years on this book, coming back to it while working on other projects, visiting researchers at labs and universities across the US, Australia, Europe, Africa, Malaysia and Bangladesh, while also spending time with field workers in tropical jungles gathering samples in ‘hot spots’, several times into the countries which comprise Central Africa, following those looking for ‘answers’ to AIDS, Ebola, Marberg, Nipa, SARS and others, visiting Chinese ‘wet markets’, even helping trap wild animals and learning about noninvasive sampling techniques, causing less disturbance to wild animals, while keeping themselves safe, or reasonably so, from infection. I’ve come away from this book with a great deal more respect for the doctors, microbiologists, veterinarians and others, doing the dangerous and essential work in the field, and doing lab work culturing ‘live’ genetic material to better understand and treat the diseases they cause, people who themselves often risk infection. I’ve also come to understand that if we are to successfully combat human diseases, and the potential of devastating ‘new’ ones, we must understand that whatever the source of the contagion, whether it is viral, bacterial, fungal, a protozoan or prion, that they exist in the larger world and have their own ‘drives’ to continue and reproduce within an ecological context, that is, in dynamic relationships with other organisms and communities.

Quammen takes the reader on a ‘tour’ through history, tracing our understanding of disease while taking us to previous world hot spots where particular disease have ‘spilled over’, with fatal consequences, as doctors and researchers, through cooperative, international effort, have been working to find the sources of these disease, how as ‘zoonotic’ diseases’, those which have existed in other species, become capable of ‘spilling over’ into susceptible human individuals and populations. This basic work is essential to understanding how various diseases, especially those viruses, which are themselves coded and reproducible by their much more ‘volatile’ RNA, greatly speeding their rates of mutation and adaptability, as an infection crosses the species boundary. Virtually ever species of organism has a specific collection of these contagions particularly adapted to ‘life’ and reproduction within them. The contagion which causes a disease must somehow be able to reproduce within its host and then successfully be transferred to the next to continue the spread, and through it, the existence of the bacteria, virus, fungi, prion etc. Their role in causing a disease within their host is secondary to their simple existence. The are a significant part of the biological communities within which they live/exist, part of the same world in which we live. They are not separate, with some destructive, evil agenda. They are role players like ourselves.

Quammen spends 100 pages discussing the work on HIV AIDS, how they’ve traced it back through genetic evidence gained through samples taken long ago, where it first arose. Anthropological work has helped ‘flesh’ out a probable story of how this disease crossed over, how given its mode of transmission, given the mores and conventions of Central Africa where it arose, combined with the practice of medicine, particularly in the first half of the 20th century the disease was likely spread and how it mutated into the various lineages, with their different degrees of virulence, that exist today. HIV AIDS is a retro-virus which rewrites the host cell’s DNA as it replicates, a much more stable and slow process in terms of mutation, especially when compared to those Viruses which rely on the very unstable RNA they use to replicate. Every disease is different, often with similar symptoms, often identifiable only by an analysis of their genetics.

A second essential point Quammen makes is that these relationships between host and disease agent, are directly effected by our actions as individuals and as larger populations, in every sense, as we alter and transform the environment, and as we attempt to either work to eradicate, or alternatively, choose to ignore, the disease and its cause. Because of the intrinsic relationships between all organisms, and their disease causing agents we cannot hope to ‘defeat’ them alone by medically treating individual patients or even through broader public health measures, though both are important. These disease agents do not exist to make us sick. The disease is an after effect. Their continuing existence is assured through the same ‘forces’ of natural selection that has resulted in every living species of organism today. They are products of evolution. In fact science has shown that infection has at times played a positive role in evolution of species, via positive ‘mutation’ of a genome. They also play a large role in maintaining a ‘healthy’ balance within the species that comprise communities. They do this much like predators do culling the weak or unfortunate, which in turn supports the health of the predators. Studies are showing that populations which are subject to pandemics tend to be those out of balance with their communities, with individuals living in relatively dense conditions, in a compromised environment which may create stresses that may make the population more susceptible to infection. In the case of human pandemics we must understand that never before have there been so many human individuals, often crowded together, who have ‘invaded’ and converted so much of the landscape where wildlife previously lived without human pressures. Today as we crowd other species, putting them under stress, in close proximity, we create the conditions where the likelihood of the ‘spillover’ of diseases is much more probable. Chance is increased by the probability of exposure and contact. Zoonotic diseases in such a world are far more likely to become human diseases. Again, their continual mutation increases the likelihood, that such a disease will mutate into something particularly transmissible and virulent to humans.

Important to the understanding of zoonotic diseases and their ability to spill over infecting human hosts, is the concept of a reservoir species, a species which can host the disease organism over those various periods of time in which there are no human cases, no outbreak. Every species has a catalog of diseases particular to it which can rarely if ever cross the species line . A disease can go for many decades within a reservoir species often going entirely unnoticed. A reservoir species may be infected but appear entirely asymptomatic. It could also have a high mortality rate. Key to such a disease’s long term existence is that in either case it reliably replicates itself and is able to move on to uninfected individuals and populations thus sustaining it into the future. In some cases the reservoir species are either too isolated from human species or shed in such small amounts that humans are rarely if ever infected. Sometimes in these cases there are intermediary species which act as amplifiers greatly increasing the amount of virus shed and bringing them into close proximity to human hosts. There are a multiplicity of factors involved, beginning with the simple fact that most of the diseases that infect other species will never infect a human host. But disease and infection are a numbers game and those numbers greatly increase as the availability of the virus and our proximity to certain species increase, increasing our own numbers and the density of our communities, creating more frequent opportunities for transmission and ‘improved’ conditions which support the rapid and effective mutation of the disease into something more dangerous for humans. Has our human population increases and we spread across the planet into areas with wild life that previously we had relatively little contact with the odds of a spill over event increase greatly. Diseases, like all species, have an evolutionary imperative to continue. As we move into its landscape reducing its options, the numbers can work in such away that a spillover event occurs. Disease organisms, have a reproduction rate that vastly outpaces ours by several orders of magnitude, increasing the likelihood of mutations that can sicken us, and, if these are based on RNA rather than DNA, an even far higher rate of mutation.

Quammen asks several researchers and disease specialists who have dedicated their lives to their study whether they think such an outcome is likely: “It all depends.’ is the last line in the book. Right now, in May of 2022, our individual and public response would indicate that so many doubt science that should a pandemic strike, which is highly transmissible and with a higher mortality rate, that it would spread rapidly with horrific results. But that is not foregone. We have the tools to protect ourselves, and if we are smart enough to continue the sampling and research around the world, we have been, and are open to its warnings, it is not inevitable. The probability of such a cataclysmic pandemic can be lessened as we move into the future and we begin to act on the understanding that we all, all species live in and have a necessary role in the world, and it benefits us all to insure the health of all species and their requisite communities. Unfortunately, our record on this has historically been poor. Still, read this book and take action, if only in your own and your family’s life. The future is not determined.

Here’s a recorded interview with the author David Quammen, done in 2020, with Scientific American as we began blundering our way through the COVID pandemic.