“It was ‘the perfect storm’—a tempest that may happen only once in a century—a nor’easter created by so rare a combination of factors that it could not possibly have been worse. "

Sebastian Junger, The Perfect Storm: A True Story of Men Against the Sea

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Most of us know the flu—influenza—as a nuisance disease, an annual annoyance to be endured along with taxes, dentists, and visits with the in-laws. Why worry about influenza when there are so many more colorfully gruesome viruses out there like Ebola? Because influenza is scientists’ top pick for humanity’s next killer plague. Up to 60 million Americans come down with the flu every year. What if it suddenly turned deadly?

H5N1, the new killer strain of avian influenza spreading out of Asia, has only killed about a hundred people as of mid-2006. In a world in which millions die of diseases like malaria, tuberculosis, and AIDS, why is there so much concern about bird flu?

Because it’s happened before. Because an influenza pandemic in 1918 became the deadliest plague in human history, killing up to 100 million people around the world. Because the 1918 flu virus was likely a bird flu virus. Because that virus made more than a quarter of all Americans ill and killed more people in 25 weeks than AIDS has killed in 25 years—yet in 1918, the case mortality rate was less than 5%. H5N1, on the other hand, has officially killed half of its human victims.

H5N1 took its first human life in Hong Kong in 1997 and has since rampaged west to Russia, the Middle East, Africa, and Europe. It remains almost exclusively a disease of birds, but as the virus has spread, it has continued to mutate. It has developed greater lethality and enhanced environmental stability, and has begun taking more species under its wing. Influenza viruses don’t typically kill mammals like rodents, but experiments have shown that the latest H5N1 mutants can kill 100% of infected mice, practically dissolving their lungs. The scientific world has never seen anything like it. We’re facing an unprecedented outbreak of an unpredictable virus.

Currently in humans, H5N1 is good at killing, but not at spreading. There are three essential conditions necessary to produce a pandemic. First, a new virus must arise from an animal reservoir, such that humans have no natural immunity to it. Second, the virus must evolve to be capable of killing human beings efficiently. Third, the virus must succeed in jumping efficiently from one human to the next. For the virus, it’s one small step to man, but one giant leap to mankind. So far, conditions one and two have been met in spades. Three strikes and we’re out. If the virus triggers a human pandemic, it will not be peasant farmers in Vietnam dying after handling dead birds or raw poultry—it will be New Yorkers, Parisians, Londoners, and people in every city, township, and village in the world dying after shaking someone’s hand, touching a doorknob, or simply inhaling in the wrong place at the wrong time.

Mathematical models suggest that it might be possible to snuff out an emerging flu pandemic at the source if caught early enough, but practical considerations may render this an impossibility. Even if we were able to stamp it out, as long as the same underlying conditions remain, the virus would presumably soon pop back up again, just as it has in the past.

This book explores what those underlying conditions are. The current dialogue surrounding avian influenza speaks of a potential H5N1 pandemic as if it were a natural phenomenon—like hurricanes, earthquakes, or even a “viral asteroid on a collision course with humanity” —which we couldn’t hope to control. The reality, however, is that the next pandemic may be more of an unnatural disaster of our own design.

Since the mid-1970s, previously unknown diseases have surfaced at a pace unheard of in the annals of medicine—more than 30 new diseases in 30 years, most of them newly discovered viruses. The concept of “emerging infectious diseases” used to be a mere curiosity in the field of medicine; now it’s an entire discipline. Where are these diseases coming from?

According to the Smithsonian Institution, there have been three great disease transitions in human history. The first era of human disease began with the acquisition of diseases from domesticated animals, such as tuberculosis, measles, the common cold—and influenza. The second era came with the Industrial Revolution of the 18th and 19th centuries, resulting in an epidemic of the so-called “diseases of civilization,” such as cancer, heart disease, stroke, and diabetes. We are now entering the third age of human disease, which started around 30 years ago—described by medical historians as the age of “the emerging plagues.” Never in medical history have so many new diseases appeared in so short a time. An increasingly broad consensus of infectious disease specialists has concluded that nearly all of the ever more frequent emergent disease episodes in the United States and elsewhere over the past few years have, in fact, come to us from animals. Their bugs are worse than their bite.

In poultry, bird flu has gone from an exceedingly rare disease to one that crops up every year. The number of serious outbreaks in the first few years of the 21st century has already exceeded the total number of outbreaks recorded for the entire 20th century. Bird flu seems to be undergoing evolution in fast-forward.

The increase in chicken outbreaks has gone hand-in-hand with increased transmission to humans. A decade ago, human infection with bird flu was essentially unheard of. Since H5N1 emerged in 1997, though, chicken viruses H9N2 infected children in China in 1999 and 2003, H7N2 infected residents of New York and Virginia in 2002 and 2003, H7N7 infected people in the Netherlands in 2003, and H7N3 infected poultry workers in Canada in 2004 and a British farmer in 2006. The bird flu virus in the Netherlands outbreak infected more than a thousand people. What has changed in recent years that could account for this disturbing trend?

All bird flu viruses seem to start out harmless to both birds and people. In its natural state, the influenza virus has existed for millions of years as an innocuous, intestinal, waterborne infection of aquatic birds such as ducks. If the true home of influenza viruses is the gut of wild waterfowl, the human lung is a long way from home. How does a waterfowl’s intestinal bug end up in a human cough? Free-ranging flocks and wild birds have been blamed for the recent emergence of H5N1, but people have kept chickens in their backyards for thousands of years, and birds have been migrating for millions.

In a sense, pandemics aren’t born—they’re made. H5N1 may be a virus of our own hatching coming home to roost. According to a spokesperson for the World Health Organization, “The bottom line is that humans have to think about how they treat their animals, how they farm them, and how they market them—basically the whole relationship between the animal kingdom and the human kingdom is coming under stress.” Along with human culpability, though, comes hope. If changes in human behavior can cause new plagues, changes in human behavior may prevent them in the future.