In the spring of 2003 I collapsed at a college debating tournament dinner and soon found myself in quarantine in Guelph, Ontario. Collapsing with a cough and fever is worrisome at any time. It was particularly troubling to me and to the intake nurse who whisked me into an isolated room because at the time I was living in Toronto, which was then one of several cities internationally caught in the grip of a frightening new respiratory disease. SARS, Severe Acute Respiratory Syndrome, was a viral lung infection that caused atypical pneumonia and was frequently fatal.

In my case, the ER doctor determined that I had a less serious virus and mild exhaustion, and released me from my HEPA-filter prison. Other patients weren't so lucky. Worldwide, SARS killed an estimated 774 of the 8,096 people infected, a fatality rate of 9.6%. The world, all things considered, was fortunate. While those deaths were tragic, they were a fraction of what was feared.

The signs in 2003 had been ominous for those familiar with medical history. SARS produced pneumonia in otherwise healthy adults, turning them blue before leaving them to drown in their own lung fluids. The threat of a pandemic felt very real.

Although SARS is a different type of virus, such frightening symptoms brought to mind one of the greatest public health nightmares of the twentieth century: the influenza pandemic of 1918-1919, the most widespread and fatal in recorded history. That pandemic of the influenza A virus killed anywhere from 50 to 100 million people worldwide, many of whom were young and healthy before being stricken. It caused widespread social and economic upheaval, and SARS looked poised to potentially do the same.

In the end, SARS was not contagious enough to cause such widespread devastation. Yet, since the SARS outbreak, the world has remained on high alert, watching nervously for a new pandemic, whether flu or some other respiratory disease.

Beginning in 2004, Asian "Bird Flu," H5N1, a deadly form of influenza, attracted much anxious attention. But, as we soon learned, the concern was misdirected. The next pandemic came not from birds but from swine and, for Americans, from much closer to home.

In spring of 2009, reports of a particularly deadly form of influenza A—H1N1—began appearing in Mexico and the American southwest. By June, the World Health Organization announced that the disease had spread widely enough to be considered pandemic. The virus spread globally, peaking, it seems, in the later months of 2009 and causing over 14,000 deaths worldwide. What the world dodged in 2003 with SARS it could not in 2009. Pandemic influenza, a frequent visitor in the past, was back again.

As the threat of the H1N1 virus began to fade (for this year at least), the world breathed a sigh of relief that we had avoided a repeat of 1918. Yet, people around the globe continue to worry and wonder how best to prepare for, perhaps even prevent, the next pandemic.

The viruses that affect humans come in many types, and their impact on human communities varies considerably according to the qualities of a particular virus—how contagious and how virulent or fatal—and the ways that human societies act or structure themselves that facilitate (or block) a virus's spread.

SARS and influenza hold a special place in pandemic planning because, like any airborne virus, they can be caught from casual contact: from a sneeze on a bus, a dirty door handle, or a shared water fountain.

Although viruses such as Ebola—which causes bleeding from the eyes, mouth, and organs—are deadlier, they require more direct contact to be transmitted and, with proper local quarantines, usually burn themselves out before they can spread.

Likewise, HIV, the virus that causes AIDS, has been pandemic for decades. HIV, however, is more difficult to transmit than airborne influenza or SARS, and requires different community-based initiatives for its treatment and prevention. The stigma that has long been associated with HIV has also allowed many to ignore the disease (at their peril) as one that happens to other people.

While diseases such as HIV and Ebola brought viruses back into the consciousness of the Western public, long accustomed to the safety afforded by antibiotics and vaccines, they represent a different public health danger than influenza and SARS.

Influenza is one of humanity's most persistent and constant foes—many will get sick each year with some form of flu, some of them will die—but only every so often does a pandemic occur. While it is impossible to prevent influenza, today we are working to reduce the chance of a pandemic and we are learning how to respond more efficiently to pandemics when they hit to avoid mass fatalities.

Influenza, unlike some epidemic diseases, has no magic bullet. As a virus, it is impervious to antibiotics. Anti-virals can help, but they do not cure. So influenza, like many viral illnesses, requires an array of strategies to deal with infection and containment. And it requires no small amount of luck. A century of advancing medical science has given humanity a variety of new weapons to fight the spread of influenza. But each has its limitations.

Going forward, to minimize or avoid the next pandemic, humans need to understand fully the patterns of human and viral behavior, and to look not only to cataclysmic pandemics like 1918 for lessons but also to the less spectacular flu pandemics that have dotted the landscape of the twentieth century.

A Very Brief History of Pandemic Influenza

Influenza is a perpetual scourge. Even though its fatality rate is relatively low (typically 0.1% deaths per infected individual), in an average year, influenza kills 250,000 to 500,000 globally according to a 2003 World Health Organization (WHO) estimate. In America, 98,000 can expect to be hospitalized, and there are, on average, 36,000 deaths, of which 82 are children. By contrast, cancer kills 500,000 Americans annually and car crashes kill 42,000.

Pandemic influenza is far deadlier. Pandemics, according to the current WHO definition, are new viruses that spread quickly, human to human, over a broad geographic area, taking advantage of a lack of prior immunity. In the popular understanding, pandemics are also considered especially sickening or deadly. (Until recently, this was approximately the way the WHO classified them as well.) In 1918, approximately 675,000 Americans lost their lives to influenza. In 2009, approximately, 8,000-16,000 Americans have died, and perhaps 60 million Americans have been sickened.

Pandemic influenza has a long history. In the 1600s, "Sweating sickness" panicked Europe, spreading in towns and royal courts across the continent. Later, similar diseases earned the name influenza—from the Italian word for influence—because they travelled so quickly that people reasoned the only way so many could become so ill so fast was if poor weather conditions or astrological forces were the cause.

The first "modern" pandemic occurred in 1890 and was called the Russian flu, because it was believed to have originated in Siberia. With new technologies like the steamship and the railway, diseases like influenza had easier ways to spread over long distances. A journey for disease that would have taken months from Moscow to London when people traveled by foot, boat, or cart, now took mere weeks, giving populations less time to prepare and less opportunity to quarantine. Luckily, the "Russian flu" was fairly mild. While many became ill, relatively few died.

"The mother of all" influenza pandemics took place in 1918, and the memory of it continues to strike the most fear into the heart of medical professionals and historians alike. In the midst of the chaos, hardship, and human displacement of World War I, an unusually deadly strain of influenza began to emerge in the spring. By the fall, after a brief lull in the outbreak, the full force of the virus began to hit. An estimated one third of the world's population were infected and showed symptoms of the illness.

"Black Octobers" and "Black Novembers" greeted many countries, when the second wave's death toll was highest. At its worst, the pandemic brought cities to their knees. Patients who were healthy one day were "gripped" by fever the next, experiencing difficulty breathing and muscle weakness. By the end of the pandemic, between 50 and 100 million people worldwide had turned blue and died (a rate of approximately 2.5% case-fatality). Gymnasiums were turned into morgues when too many died to fit into hospitals or cemeteries.

No one is quite sure why the 1918 pandemic turned out so badly. With World War I still raging, people in Europe were hungry and stretched to the breaking point, making them more susceptible to infection. Peace-time social services were no longer operating and many people lived in crowded and unsanitary conditions. Troops, migrant laborers, and refugees spread the virus rapidly across vast distances.

Even the end of the war had its dangers. When the Armistice was signed in November, 1918 many communities saw an uptick in flu cases as happy and relieved citizens celebrated together, sharing good wishes and influenza at parades and parties. A third, less deadly wave appeared in the spring before the epidemic died down in mid-1919.

After 1918, the world had a chance to catch its breath. In fact, the severity of 1918 seemed to provide some immunity against later infections, for a little while at least. In 1957-1958 and 1968-1969, however, there were two "minor" pandemics—minor at least when compared to 1918.

The 1957 pandemic began in Guizhou, China, then spread to Singapore, Hong Kong and moved into the United States by the fall, where it killed an estimated 70,000 people. Total deaths globally were between 1.5 to 2 million people.

Many people in 1957 still remembered the 1918 pandemic and were on edge. In an effort to avoid panic, the publication of a book commemorating the fortieth anniversary of the 1918 pandemic, called "Invasion by Virus: Can it Happen Again?" was delayed when the answer had clearly become "yes."

In 1968, another virus began, we think, in Asia. The "Hong Kong flu" was marked by high morbidity but relatively low fatality—that is, many got sick but few died. Only 34,000 Americans were killed and an estimated one million worldwide. Many older people were unaffected, probably because they had caught and survived the similar 1918 influenza.

Despite the low death rates, such frequent flu pandemics have made the world anxious. In 1976, when swine flu was detected at Fort Dix Army Base in New Jersey, health officials sounded the alarm. The 1918 pandemic had been a swine flu as well, and they worried that after the "near misses" of 1957 and 1968 that the world was due for another nightmarish pandemic. That pandemic didn't materialize, but it did generate a large-scale American vaccine program, which has shaped many our modern anti-influenza measures.

Modern Responses

Death tolls for the present pandemic are mercifully lower than for 1918. It seems, perhaps, that the virus is less lethal. There is no world war that has weakened the population and basic sanitary situations have improved in many parts of the world. Thanks to a century of medical research, we have better respirators, anti-virals and vaccines that treat or prevent influenza directly. We also have antibiotics that, with luck, help to counteract the bacterial "purulent bronchitis"—a form of pneumonia that develops in the fluid in the lungs of an influenza sufferer—that carried away many of 1918's victims.