Quammen is a writer, not a scientist, and the book reads more like a memoir of his travels and encounters (and sometimes as a novel) than an (informative) piece of science. The book is structured as a collection anecdotes. It takes many pages of narrative to get to a conclusion. If you are just looking for a simple answer to a simple question like "how did SARS emerge"? you'll be better off searching the web than reading the many pages about how SARS was discovered and studied. But the sheer amount of on-site documentary reportage is astounding. Quammen was a man on a mission, determined to document how diseases were discovered and studied. He often behaves like a detective. His scientists are introduced almost as expert witnesses in a trial. Along the way we get to meet literally hundreds of scientists, past and present, and Quammen describes many of his personal encounters and adventures with them.

Zoonosis is the process by which a virus jumps from an animal to a human. The flu, ebola, AIDS, SARS and covid are all examples of zoonotic diseases. Smallpox is not zoonotic. It is relatively easy to destroy a non-zoonotic disease because it's only "reservoir" is humans: cure all humans and the disease disappears. A zoonotic disease is harder to eradicate because the originating virus hides in some other animal, and in the vast majority of cases it is not a domestic animal: the deadliest zoonotic diseases have been spread by wild animals. As humans invade the habitats of wild animals, transmission becomes more likely. As humans travel faster and longer distances, infectious diseases spread faster and over longer distances. One problem with the covid pandemic was that, by the time people understood that it was a deadly virus, the virus had already spread to many corners of the planet. Ebola, which is deadlier, has not spread as widely because it was first identified in a place where people didn't take international flights, high-speed trains and not even car trips.

Part 2 of the book deals with ebola, which first emerged in 1976. Eric Leroy thinks that ebola is (and has been) ubiquitous in some forests of Africa and occasionally jump to humans through contact with wildlife. Peter Walsh thinks that ebola is a new virus that is expanding from its source like a wave. Nobody has made sense yet of the 35 ebola outbreaks from 1976 to 2022. Nobody knows what the original transmitter of ebola is. We only know that fruit bats are "healthy" carriers, i.e. they spread the virus without getting sick.being affected by it.

Part 3 of the book is about malaria, which is spread by mosquitoes. Part 4 is about SARS, which is interesting as a forerunner of covid. It too is casued by a coronavirus. Guan Yi determined that the wet markets were the places where SARS most likely jumped from animals (most likely civet cats) to humans. In 2013 (after the publication of this book) the famous Wuhan scientist Zhengli Shi found its closest relative SL-CoV WIV1 in horseshoe bats, which are therefore considered to be a reservoir of SARS (there might be others). In 2017 Chinese scientists found all the genetic building blocks of SARS in a single population of horseshoe bats.

Part 5 is mostly about Lyme disease. Part 6 is one of the most interesting. It begins with Herpes-B but then deals with RNA viruses. Today (and not necessarily in the past), RNA viruses are the most common class of pathogens: the influenzas, ebola, West Nile, dengue, yellow fever, rabies, HIV, SARS, covid... Edward Holmes' "The Evolution and Emergence of RNA Viruses" (2009) points at simple factors to explain why RNA viruses seem to proliferate and be so effective at making us sick. RNA are error-prone fast replicators: they replicate rapidly but they make mistakes all the time, resulting in huge rates of mutation, and this gives them the ability to adapt easily, including the ability to jump species. They are bound by their genetic instability to have small genomes (the Manfred Eigen paradox, the bigger the mutation rate the smaller the genome). DNA virus, on the contrary, have generally bigger genomes and are very stable, but can't adapt easily.

Part 7 is one of the most interecting parts because it is about bats (chiroptera). If you just list all the zoonotic viruses that hurt us, you will notice that so many come from bats, and don't hurt bats: rabies (the first one discovered in rabies, back in 1955), dengue, West Nile, Zika, encephalitis, hantavirus, Nipah virus, Hendra virus, Marburg virus, MERS, SARS and most likely ebola, even if other hosts, such as civets for SARS and camels for MERS are proximate reservoirs for human infection. This fact was first recognized by Charles Calisher's 2006 paper "Bats - Important Reservoir Hosts of Emerging Viruses". (This book was published before the covid pandemic, and in any case it's too early to draw conclusions about covid, but a bat virus is yet again the closest relative to the covid virus). To start with, bat innate immunity means that there is something important that is completely different in the immune system of bats, which may be due to the fact that bats are a very old mammal lineage. They also live in huge communities. It is not unusual for mammals to live in large communities (think of herds of bisons and zebras) but bat communities are the only ones that are segregated in closed environments like caves. And then it's obvious what makes bats unique among mammals: bats are the only flying mammals. After the invention of the hot-air balloon, humans became the second. (See also Michael Letko's 2020 paper "Bat-borne virus diversity, spillover and emergence" and Xinsheng Yao's "Severe zoonotic viruses carried by different species of bats and their regional distribution" of 2023). Quammen correctly asks also the opposite question: if bats harbor so many viruses and they can easily spread them to humans, why don't we have more epidemics? Maybe human-to-human transmission requires specific conditions that are not always present.

Part 8 is the story of how AIDS was identified and speculations on how it may have started. There was a "conspiracy theory", originating from an article written by Tom Curtis in 1992 in Rolling Stone magazine and amplified by Edward Hooper's one-thousand page book "The River", that the massive polio vaccination of 1958 helped spread the HIV because nurses were using contaminated syringes. That one was most likely false but the real story might be not too different. Quammen reconstructs how scientists determined that the spillover of HIV from chimps to humans occurred before 1908 (Michael Worobey's 2008 paper "Direct evidence of extensive diversity of HIV-1 in Kinshasa by 1960") in south Cameroon ("Chimpanzee reservoirs of pandemic and nonpandemic HIV-1" of 2006 is the paper that pinpointed the area). Quammen speculates that a chimp hunter in south Cameroon was patient zero and then someone traveled from that area to Kinshasa around 1920, where it kept mutating until it became the deadly HIV-1 group M. The earliest known sample is from 1959 (hence Curtis' conspiracy theory). Jacques Pepin investigated how the virus could have survived, spread and evolved in Congo and published the results in the 2012 book "The Origins of AIDS". He thinks that (almost borrowing from Curtis' theory) the culprit were the medical campaigns started in the 1920s by the Belgian colonists to eliminate tropical diseases. Those did entail injections (e.g. injecting quinine to fight malaria) at a time when disposable syringes were a luxury. Around 1966 the virus jumped to another continent: Haiti. Pepin speculates that it was the indirect consequence of the independence of Congo, attained in 1960. An exodus of white European doctors, teachers and other professionals began just before 1960. After independence the government of Congo went looking for replacements and found them in Haiti, which shared a language and a colonial past with Congo. Mobutu seized power in 1963 and the Haitians began returning home. Around 1966 a returning Haitian brought the virus to Haiti ("The emergence of HIV/AIDS in the Americas and beyond" is the 2007 paper that pinpointed the date) and there it started multiplying faster than it had in Africa. (Surprisingly, Quammen doesn't ask the obvious question: why HIV was so lethal in the Americas and not so lethal in its original Congo homeland?) Pepin then investigated how a virus could travel from Haiti to San Francisco and discovered that in 1971-72 a Miami-owned company called Hemo Caribbean was shipping blood plasma from Haiti to the USA, but that's very weak circumstantial evidence. The virus may have arrived earlier in the USA. In 1981 a 33-year-old UCLA professor, Michael Gottlieb, realized that some homosexual patients were suffering from a new deadly disease and the following year that disease got the name AIDS.

One lesson learned from the story of AIDS is that it surfaced in California, but it had actually come from Haiti, and originate in Africa. Similarly, the "Spanish Flu" did not originate in Spain but in the USA. The place where it is identified is not always the place where it originated.

The book deals with influenza only at the end, and briefly mentions the "Spanish Flu" (H1N1 virus), the 1957 "Asian Flu" (H2N2) and the 1968 pandemic (H3N2 virus). It also repeats the warning that the "next big one" is coming and it will probably be zoonotic and probably a relative of the flu. Seven years later the book's warning covid started killing people in Wuhan.

Jon Epstein thinks that these viruses have existed in the wild for a long time and their "carriers" have become immune by natural selection. Viruses jump from species to species, and who knows in how many cases they make no difference or they even turn out to be benign (see Karin Moelling's "Viruses - More Friends Than Foes"). We have no idea how many times a virus jumped from an animal to a human and nothing bad happened. We only become aware of "spillovers" that cause epidemics.

If many of these diseases have existed for a long time and have been confined for a long time in their ecological niches, killing only occasionally the unlucky ones who ventured into those ecological niches, the modern world is moving towards what William McNeill called "disease homogenization": those old localized diseases will slowly become global diseases shared by all regions of the world. The first "globalized" human who gets infected by a niche virus turns that virus into a planetary pandemic. We may soon have to confront all harmful viruses in all regions of the world.

The issue that "Spillover" does not confront is how to get rid, once and forever, of zoonotic diseases. So far we've only come up with temporary solutions. Vaccines boost the immune system so it can fight viruses, but viruses keep mutating and multiplying all over the planet. Antibiotics kill bacteria inside the body, but the number of bacteria is virtually infinite. And for all the diseases spread by mosquitoes the first and foremost strategy is to simply avoid being bitten. Diseases are spreading faster because of globalization (people travel for business all the time), urbanization (people live in denser areas), tourism (people travel for fun to very different ecosystems and), overpopulation (there are people everywhere) and climate change (ecosystems are being disrupted sending microorganisms and their carriers to migrate into new regions). These trends will only accelerate. Tourism, in particular, is likely to become a truly "mass" and "global" phenomenon, as the middle class expands in all countries of the world. For example, Zika originated in Africa in the 1950s and caused small epidemics in Oceania between 2007 and 2014 but it was imported into Brazil by a traveler from French Polynesia, and in Brazil it caused a much bigger epidemic. Covid itself was probably exported from China to the USA by a traveler returning from visiting family in Wuhan, and probably exported by Western tourists to African and Latin American regions. AIDS also showed the effect that patterns of social behavior can have on diseases: AIDS would not have spread the way it did in the USA without the sexual revolution of the 1960s and the spread of addictive drugs. Sexual promiscuity (in particular amongs gays) and intravenous injections of heroin (via shared syringes) colluded to create the perfect vehicle for the HIV. Can we solve the problem upstream? Mosquitoes cause malaria, West Nile, Zika, yellow fever, dengue (one of the fastest spreading diseases). Since the invention of DDT, the temptation has been there to "control" the number of mosquitoes, i.e. to exterminate them. Rats cause plague and hantavirus. Rat eradication programs have been launched in many countries. Bats carry Marburg, ebola, SARS, covid, Nipah, Hendra and others. But bat extermination programs don't exist because bats play an important role in the ecosystem, and in some countries they are even protected by law. However, one can imagine a world without mosquitoes, rats and bats, a world in which diseases like malaria, plague, ebola and covid would probably not exist. Not many people would be sorry if all mosquitoes and rats are killed, and relatively few are fond of bats, despite the fact that we have to be grateful to rats for many therapies that were discovered by experimenting on rats, and despite the fact that bats control the population of insects like beetles and moths and pollinate tropical plants. And what if some day a deadly virus started spreading among the human race's favorite pets, dogs and cats? Would we be willing to exterminate our dogs and cats to save human lives? Other than exterminating the zoonotic carriers, is there another "final solution" to get rid once and forever of zoonotic diseases?

If the book doesn't offer powerful scientific arguments, it's just because we still know very little about how viruses originate and jump from animals to us. That's ultimately the message of this thick 600-page book: we know close to nothing about viruses, the virosphere, how they spread, how they interact with us. In fact, we still don't know how to "kill" them: a vaccine is simply a way to boost our immune system, and then hope that the boosted immune system will resist the attacker. We never invented the equivalent of an antibiotic (that kills bacteria). We are still largely helpless against viruses, convinced that we are the smartest being on Earth while viruses multiply and kill us by the millions.

Finally, a philosophical consideration. From the viewpoint of other animals, a major, lethal, disease is human civilization: humans eat billions of mammals, birds and fish every year, not to mention all the insects and spiders that we spray to death.

TM, ®, Copyright © 2024 Piero Scaruffi