A Planet of Viruses

A Planet of Viruses

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by Carl Zimmer

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Viruses are the smallest living things known to science, yet they hold the entire planet in their sway. We are most familiar with the viruses that give us colds or the flu, but viruses also cause a vast range of other diseases, including one disorder that makes people sprout branch-like growths as if they were trees. Viruses have been a part of our lives for so

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Viruses are the smallest living things known to science, yet they hold the entire planet in their sway. We are most familiar with the viruses that give us colds or the flu, but viruses also cause a vast range of other diseases, including one disorder that makes people sprout branch-like growths as if they were trees. Viruses have been a part of our lives for so long, in fact, that we are actually part virus: the human genome contains more DNA from viruses than our own genes. Meanwhile, scientists are discovering viruses everywhere they look: in the soil, in the ocean, even in caves miles underground.

This fascinating book explores the hidden world of viruses—a world that we all inhabit. Here Carl Zimmer, popular science writer and author of Discover magazine’s award-winning blog The Loom, presents the latest research on how viruses hold sway over our lives and our biosphere, how viruses helped give rise to the first life-forms, how viruses are producing new diseases, how we can harness viruses for our own ends, and how viruses will continue to control our fate for years to come. In this eye-opening tour of the frontiers of biology, where scientists are expanding our understanding of life as we know it, we learn that some treatments for the common cold do more harm than good; that the world’s oceans are home to an astonishing number of viruses; and that the evolution of HIV is now in overdrive, spawning more mutated strains than we care to imagine.

The New York Times Book Review calls Carl Zimmer “as fine a science essayist as we have.” A Planet of Viruses is sure to please his many fans and further enhance his reputation as one of America’s most respected and admired science journalists.

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Editorial Reviews

"Part of a series sponsored by the Science Education Partnership Award (SEPA) to help support educational outreach to students, [A Planet of Viruses] packs into 109 pages just about everything you’ve always wanted to know–and a lot you’ll probably wish you didn’t know–about the viruses that have caused humanity so much grief throughout history."—Forbes


Times Higher Education

"For those with long memories, not much seems to have happened in fundamental physics and cosmology since Carl Sagan's Cosmos, 30 years ago. . . . The real action is in biology, where amazing new facts just keep coming. The techniques of genome analysis make it remarkably easy at the moment to make unexpected observations. [A Planet of Viruses] is packed with them, carefully assembled by another talented populariser, the science writer and Yale University lecturer Carl Zimmer."—Times Higher Education
Onion A.V. Club
"Science writer Carl Zimmer has a penchant for writing about things most humans like to avoid; his previous works include Microcosm: E. Coli And The New Science Of Life, and Parasite Rex. Each chapter of his latest work is dedicated to a different type of virus, providing a brief synopsis on what makes a certain species unique, and using the example to launch into fascinating information about what it teaches about the nature of viruses and life in general."
"Zimmer’s information-packed, superbly readable look at virological knowledge awakens readers to the fact that not only are viruses everywhere but we couldn’t live without them."—Booklist

Boing Boing
Carl Zimmer is one of my absolute favorite science writers, and he's about to come out with a new book called A Planet of Viruses. I'm a bit of a virology fangirl, and am, thus, ridiculously excited about this news.”

— Maggie Koerth-Baker

Lancet Infectious Diseases

“A contagious fear pervades the public perception of viruses, and rightly so, because they cause many serious diseases; but they are not all bad. In A Planetof Viruses Carl Zimmer seeks to convey this message, elegantly communicating the history of viruses, their symbiotic relation with life, and their influence on mankind’s development.”

“Although most everyone is familiar with the word "viruses," few people are aware of the major role they play as powerful agents of change on Earth. Zimmer presents an intriguing journey into the world of viruses, providing a fascinating historical perspective. . . . This is an insightful book that serves as an excellent resource for understanding viruses and their relationship to humans. . . . Highly recommended.”
Booklist (starred review)

“Absolutely top-drawer popular science writing. . . . Zimmer’s information-packed, superbly readable look at virological knowledge awakens readers to the fact that not only are viruses everywhere but we couldn’t live without them."


"As with any great journey, this virtual tour opens your eyes and expands your horizons. You’ll learn amazing facts. But this is no textbook. Zimmer does not do boring or stuffy; reading his work is like hanging out with the smartest, most interesting guy you have ever met as he regales you with tales of his travels and fascinating finds along the way."—ScienceNews

Washington Post

“In A Planet of Viruses, science writer Carl Zimmer accomplishes in a mere 100 pages what other authors struggle to do in 500: He reshapes our understanding of the hidden realities at the core of everyday existence. . . . Whether he’s exploring how viruses come to America or picking apart the surprisingly complicated common cold, Zimmer’s train of thought is concise and illuminating.”
Microbe Magazine

“This book is pure reading pleasure. It is amazing how seamlessly Carl Zimmer tells the stories of viruses in short chapters, describing the history, microbiology, and impacts of viruses in interesting, informative, readable chapters.”
Boing Boing - Maggie Koerth-Baker

“A smart, beautiful, and somewhat demented picture book that's likely to give you a case of the willies. In the best way possible."

Rebecca Skloot

“Carl Zimmer is one of the best science writers we have today. A Planet of Viruses is an important primer on the viruses living within and around all of us—sometimes funny, other times shocking, and always accessible. Whether discussing the common cold and flu, little-known viruses that attack bacteria or protect oceans, or the world’s viral future as seen through our encounters with HIV or SARS, Zimmer’s writing is lively, knowledgeable, and graced with poetic touches.”

Richard Preston

“I’m a serious fan of Carl Zimmer, and A Planet of Viruses provided a new treat. It’s thoughtful, precise, and engrossing, page by page. Zimmer has an uncanny ability to tell cool tales about nature that leave you with new thoughts and understanding, always keeping precisely to the science.”

Jonathan Weiner

“This little book will interest anyone on this planet who has ever played host to a virus. It is beautifully clear, eminently sensible, and fascinating from beginning to end—like everything Carl Zimmer writes. I don’t know how Zimmer does it! Neither does anyone else who follows and enjoys his work.”

Nathan Wolfe

“An accessible and gripping narrative on a serious topic that manages to explain, in plain English, how viruses are changing the world. Carl Zimmer has found great stories and woven them into an honest, optimistic book. It is a wonderfully vivid and compelling read.”

Julia Sweeney
“I hope Carl Zimmer lives a long, long time so we can get more and more books from him. . . . [A Planet of Viruses is] a short read . . . but intense and well explained.”

A.V. Club

"Science writer Carl Zimmer has a penchant for writing about things most humans like to avoid; his previous works include Microcosm: E. Coli And The New Science Of Life, and Parasite Rex. Each chapter of his latest work is dedicated to a different type of virus, providing a brief synopsis on what makes a certain species unique, and using the example to launch into fascinating information about what it teaches about the nature of viruses and life in general."
James Norton
…Carl Zimmer accomplishes in a mere 100 pages what other authors struggle to do in 500: He reshapes our understanding of the hidden realities at the core of everyday existence…Zimmer's train of thought is concise and illuminating.
—The Washington Post

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Product Details

University of Chicago Press
Publication date:
Edition description:
New Edition
Product dimensions:
5.60(w) x 8.60(h) x 0.60(d)

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A Planet of VIRUSES

By Carl Zimmer

The University of Chicago Press

Copyright © 2011 Board of Regents of the University of Nebraska
All right reserved.

ISBN: 978-0-226-98335-6

Chapter One


The Uncommon Cold


Around 3,500 years ago, an Egyptian scholar sat down and wrote the oldest known medical text. Among the diseases he described in the so-called Ebers Papyrus was something called resh. Even with that strange sounding name, its symptoms—a cough and a flowing of mucus from the nose—are immediately familiar to us all. Resh is the common cold.

Some viruses are new to humanity. Other viruses are obscure and exotic. But human rhinoviruses—the chief cause of the common cold, as well as asthma attacks—are old, cosmopolitan companions. It's been estimated that every human being will spend a year of his or her life lying in bed, sick with colds. The human rhinovirus is, in other words, one of the most successful viruses of all.

Hippocrates, the ancient Greek physician, believed that colds were caused by an imbalance of the humors. Two thousand years later, the physiologist Leonard Hill argued in the 1920s that they were caused by walking outside in the morning, from warm to cold air. The first clue to the true cause of colds came when Walter Kruse, a German microbiologist, had a snuffly assistant blow his nose and mix the mucus into a salt solution. Kruse and his assistant purified the fluid through a filter and then put a few drops into the noses of twelve of their colleagues. Four of them came down with colds. Later, Kruse did the same thing to thirty-six students. Fifteen of them got sick. Kruse compared their outcomes to thirty-five people who didn't get the drops. Only one of the drop-free individuals came down with a cold.

Kruse's experiments made it clear that some tiny pathogen was responsible for the cold. At first, many experts believed it was some kind of bacteria, but Alphonse Dochez ruled that out in 1927. He filtered the mucus from people with colds, the same way Beijerinck had filtered tobacco plant sap thirty years before, and discovered that the bacteria-free fluid could make people sick. Only a virus could have slipped through Dochez's filters.

It took another three decades before scientists figured out exactly which viruses had slipped through. Known as human rhinoviruses (rhino means nose), they are remarkably simple, with only ten genes apiece. (We have twenty thousand.) And yet that haiku of genetic information is enough to let the human rhinovirus invade our bodies, outwit our immune system, and give us colds.

The human rhinovirus spreads by making noses run. People with colds wipe their noses, get the virus on their hands, and then spread the virus onto door knobs and other surfaces they touch. The virus hitches onto the skin of other people who touch those surfaces and then slips into their body, usually though their nose. Rhinoviruses can invade the cells that line the interior of the nose, throat, or lungs. They trigger the cells to open up a trapdoor through which they slip. Over the next few hours, a rhinovirus will use its host cells to make copies of its genetic material and protein shells to hold them. The host cell then rips apart, and the new virus escapes.

Rhinoviruses infect relatively few cells, causing little real harm. So why can they cause such miserable experiences? We have only ourselves to blame. Infected cells release special signaling molecules, called cytokines, which attract nearby immune cells. Those immune cells then make us feel awful. They create inflammation that triggers a scratchy feeling in the throat and leads to the production of a lot of mucus around the site of the infection. In order to recover from a cold, we have to wait not only for the immune system to wipe out the virus but also to calm itself down.

The Egyptian author of the Ebers papyrus wrote that the cure for resh was to dab a mixture of honey, herbs, and incense around the nose. In seventeenth-century England, cures included a blend of gunpowder and eggs and of fried cow dung and suet. Leonard Hill, the physiologist who believed a change of temperature caused colds, recommended that children start their day with a cold shower. Today, doctors don't have much more to offer people who get colds. There is no vaccine. There is no drug that has consistently shown signs of killing the virus. Some studies have suggested that taking zinc can slow the growth of human rhinoviruses, but later studies failed to replicate their results.

In fact, some treatments for the cold may be worse than the disease itself. Parents often give their children cough syrup for colds, despite the fact that studies show it doesn't make people get better faster. But cough syrup also poses a wide variety of rare yet serious side effects, such as convulsions, rapid heart rate, and even death. In 2008, the Food and Drug Administration warned that children under the age of two—the people who get colds the most—should not take cough syrup.

Another popular treatment for the cold is antibiotics, despite the fact that they only work on bacteria and are useless again viruses. In some cases, doctors prescribe antibiotics because they're not sure whether a patient has a cold or a bacterial infection. In other cases, they may be responding to pressure from worried parents to do something. But unnecessary prescriptions of antibiotics are a danger to us all, because they foster the evolution of increasingly drug-resistant bacteria in our bodies and in the environment. Failing to treat their patients, doctors are actually raising the risk of other diseases for everyone.

One reason the cold remains incurable may be that we've underestimated the rhinovirus. It exists in many forms, and scientists are only starting to get a true reckoning of its genetic diversity. By the end of the twentieth century, scientists had identified dozens of strains, which belonged to two great lineages, known as HRV-A and HRV-B. In 2006, Ian Lipkin and Thomas Briese of Columbia University were searching for the cause of flu-like symptoms in New Yorkers who did not carry the influenza virus. They discovered that a third of them carried a strain of human rhinovirus that was not closely related to either HRV-A or HRV-B. Lipkin and Briese dubbed it HRV-C, and since then, researchers have found that this third lineage is common around the world. From one region to another, the variations in HRV-C's genes are few, which suggests that the virus wasted no time spreading through our species. In fact, the common ancestor of all HRV-C may be just a few enturies old.

The more strains of rhinoviruses scientists discover, the better they come to understand their evolution. All human rhinoviruses share a core of genes that have changed very little as the viruses have spread around the world. Meanwhile, a few parts of the rhinovirus genome are evolving very quickly. These regions appear to help the virus avoid being killed by our immune systems. When our bodies build antibodies that can stop one strain of human rhinovirus, other strains can still infect us because our antibodies don't fit on their surface proteins. Consistent with this hypothesis is the fact that people are typically infected by several different human rhinovirus strains each year.

The diversity of human rhinoviruses makes them a very difficult target to hit. A drug or a vaccine that attacks one protein on the surface of one strain may prove to be useless against others that have a version of that protein with a different structure. If another strain of human rhinovirus is even a little resistant to such treatments, natural selection can foster the spread of new mutations, leading to much stronger resistance.

Despite the diversity of rhinoviruses, some scientists are optimistic that they can develop a cure for the common cold. The fact that all strains of human rhinoviruses share a common core of genes suggests that the core can't withstand mutations. In other words, viruses with mutations in the core die. If scientists can figure out ways to attack the rhinovirus core, they may be able to stop the disease. One promising target is a stretch of genetic material in rhinoviruses that folds into a loop shaped like a clover leaf. Every rhinovirus scientists have studied carries the same cloverleaf structure, which appears to be essential for speeding up the rate at which a host cell copies rhinovirus genes. If scientists can find a way to disable the clover leaf, they may be able to stop every cold virus on earth.

But should they? Human rhinoviruses certainly impose a burden on public health, not just by causing colds but by opening the way for more harmful pathogens. But the human rhinovirus itself is relatively mild. Most colds are over in a week, and 40 percent of people who test positive for rhinoviruses suffer no symptoms at all. In fact, human rhinoviruses may offer some benefits to their human hosts. Scientists have gathered a great deal of evidence that children who get sick with relatively harmless viruses and bacteria may be protected from immune disorders when they get older, such as allergies and Crohn disease. Human rhinoviruses may help train our immune systems not to overreact to minor triggers, instead directing their assaults to real threats. Perhaps we should not think of colds as ancient enemies but as wise old tutors.

Looking Down from the Stars

Influenza Virus

Influenza. If you close your eyes and say the word aloud, it sounds lovely. It would make a good name for a pleasant, ancient Italian village. Influenza is, in fact, Italian (it means influence). It is also, in fact, an ancient name, dating back to the Middle Ages. But the charming connotations stop there. Medieval physicians believed that stars influenced the health of their patients, sometimes causing a mysterious fever that swept across Europe every few decades. And ever since, influenza has raged through our species. In 1918, a particularly virulent outbreak of the flu killed an estimated fifty million people. Even in years without an epidemic, influenza takes a brutal toll. Each winter, thirty-six thousand people die of the flu in the united States alone; somewhere between a quarter million and a half million people die worldwide. Today scientists know that influenza is not the work of the heavens, but of a microscopic virus. Like cold-causing rhinoviruses, influenza viruses manage to wreak their harm with just ten genes. They spread in the droplets sick people release with their coughs, sneezes, and running noses. A new victim may accidentally breathe in a virus-laden droplet or pick it up on a doorknob and then bring now-contaminated fingers in contact with their mouth. Once a flu virus gets into the nose or throat, it can latch onto a cell lining the airway and slip inside. As flu viruses spread from cell to cell in the lining of the airway, they leave destruction in their wake. The mucus and cells lining the airway get destroyed, as if the flu viruses were a lawn mower cutting grass.

In healthy people, the immune system is able to launch a counterattack in a matter of days. In such cases, the flu causes a wave of aches, fevers, and fatigue, but the worst of it is over within a week. In a small fraction of its victims, the flu virus opens the way for more serious infections. Normally, the top layer of cells serves as a barrier against a wide array of pathogens. The pathogens get trapped in the mucus, and the cells snag them with hairs, swiftly notifying the immune system of intruders. Once the influenza lawnmower has cut away that protective layer, pathogens can slip in and cause dangerous lung infections, some of which can be fatal.

For a virus that has caused so much death in the past, and which continues to claim so many victims each year, influenza virus remains surprisingly mysterious. Seasonal flu is most dangerous for people with weak immune systems that can't keep the virus in check—particularly young children and the elderly. But in flu pandemics, like the 1918 outbreak, people with strong immune systems proved to be particularly vulnerable. Scientists don't know why the flu switches targets this way. One theory holds that certain strains of the flu provoke the immune system to respond so aggressively that it ends up devastating the host instead of wiping out the virus. But some scientists doubt this explanation and think the true answer lies elsewhere. Scientists also don't know when influenza viruses first started making people sick. There certainly are historical records of outbreaks of deadly fevers going back thousands of years, but it's impossible to know whether influenza viruses caused them, or another species of virus with similar symptoms.

Amidst all the mysteries of the flu, the origin of the virus is clear. It came from birds. Birds carry all known strains of human influenza viruses, along with a vast diversity of other flu viruses that don't infect humans. Many birds carry the flu without getting sick. Rather than infecting their airways, flu viruses typically infect the guts of birds; the viruses are then shed in bird droppings. Healthy birds become infected by ingesting virus-laden water.

Sometimes strains of bird flu jump the species barrier and become human viruses. But for every successful transition, there are probably many failed crossings. Bird flu viruses are well adapted to infecting their avian hosts and reproducing quickly inside them. Those adaptations make them ill-suited to spreading among humans. Starting in 2005, for example, a strain of flu from birds called H5N1 began to sicken hundreds of people in southeast Asia. It is much deadlier than ordinary strains of seasonal flu, and so public health workers have been tracking it closely and taking measures to halt its spread. For now, at least, H5N1 can only move from a bird to a human; it cannot move from one human to another.

Unfortunately, a poorly adapted flu virus can evolve into a well-adapted one. Flu viruses are particularly sloppy at replicating their genes, so many new viruses acquire mutations. These mutations are like random changes to the letters in the flu's recipe. Some of the mutations have no effect on viruses. Some leave them unable to reproduce. But a few mutations give flu viruses a reproductive advantage. Natural selection favors these beneficial mutations, and flu strains can become better at infecting humans as mutation after mutation accumulates. Some mutations help the virus by altering the shape of the proteins that stud the virus shell, allowing them to grab human cells more effectively. Other mutations help the flu virus cope with human body temperature, which is a few degrees cooler than that of birds.

Human influenza viruses have also adapted to a new route from host to host. In birds, the viruses travel from guts to water to guts. In people, the virus moves from airways to droplets to airways. This new route also causes the flu rise and fall with the seasons. In places like the united states, most flu cases occur during the winter. According to one hypothesis, this is because the air is dry enough in those months to allow virus-laden droplets to float in the air for hours, increasing their chances of encountering a new host. In other times of the year, the humidity causes the droplets to swell and fall to the ground.

When a flu virus hitches a ride aboard a droplet and infects a new host, it sometimes invades a cell that's already harboring another flu virus. And when two different flu viruses reproduce inside the same cell, things can get messy. The genes of a flu virus are stored on eight separate segments, and when a host cell starts manufacturing the segments from two different viruses at once, they sometimes get mixed together. The new offspring end up carrying genetic material from both viruses. This mixing, known as reassortment, is a viral version of sex. When humans have children, the parents' genes are mixed together, creating new combinations of the same two sets of DNA. Reassortment allows flu viruses to mix genes together into new combinations, as well.

As scientists get a closer look at the genes of flu viruses, they're discovering that reassortment has played a major role in the natural history of the flu. A quarter of all birds with the flu have two or more virus strains inside them at once. The viruses swap genes through reassortment, and as a result they can move easily between bird species. And sometimes, on very rare occasions, an avian influenza virus can pick up human influenza virus genes through reassortment. That can be a recipe for disaster, because the new strain that results can easily spread from person to person. And because it has never circulated among humans before, no one has any defenses that could slow the new strain's spread.


Excerpted from A Planet of VIRUSES by Carl Zimmer Copyright © 2011 by Board of Regents of the University of Nebraska. Excerpted by permission of The University of Chicago Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Meet the Author

Carl Zimmer is a lecturer at Yale University, where he teaches writing about science and the environment. He is the author of numerous books, including Microcosm; Parasite Rex; Evolution: The Triumph of an Idea; At the Water’s Edge; and Soul Made Flesh. His numerous essays and articles on the life sciences have appeared in the pages of the New York Times, Scientific American, Discover, Time, Science, Popular Science, and National Geographic. His work has been anthologized in both The Best American Science Writing and The Best American Science and Nature Writing series.

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