Mexico, 2009: Citizens wear face masks during H1N1 outbreak. [Photograph: Eneas De Troya via Flickr]
Front Studio, Q-City Project, from Landscapes of Quarantine, Storefront for Art and Architecture. [Courtesy of Geoff Manaugh]
Snow's findings soon influenced urban architecture and engineering. In New York City in the 1860s, cholera and malaria outbreaks led to the establishment of the Metropolitan Board of Health — the first municipal health authority in the United States — and to building and zoning codes that controlled overcrowding, mandated better sanitary conditions, and propelled infrastructure investments that have influenced city systems and services to this day.  Indeed, those codes have served us so well over the last century and a half that we have possibly become complacent about new public health threats in cities around the world. As Manaugh and Twilley write, “Quarantine has re-emerged as an issue of urgency and importance in today’s era of globalization, antibiotic resistance, emerging diseases, pandemic flu, and bio-terrorism.”
Some statistics. The United Nations has recorded a one-third increase in global epidemics in just one year, between 2004 and 2005, and an 80-fold increase in pandemics over the 20th century.  The UN data also show that the geography of illness remains highly uneven. The number of people killed by biological illness was one-third more in the least developed countries than in developing nations and 160 times more than in the most developed parts of the world. But given the speed with which viral infections can be transported around the globe, geographical differences will matter less and less. In the 21st century the human population of the planet will likely become almost equally exposed to pandemic disease,
To be sure, the relative mildness of the 2009 H1N1 influenza outbreak might lead us to dismiss the severity of the threat. Many did die from H1N1 — nearly 5,000 globally according to the World Health Organization — but this was not enough to prompt a real increase in preventative measures.  We may not be so lucky next time. Viral infections like the Ebola and West Nile fevers kill quickly and no vaccine yet exists to fight them.  And North Americans have only to read the history of the continent for evidence of how viral infections can devastate populations: the number of native Americans declined as much as 80 percent after their encounter with unfamiliar and incurable European illnesses, although scholars still debate the method and rate at which that occurred. 
Bird flu control measures. [Photograph: Adam Burt via Flickr]
What does it mean to design with viruses in mind today? We might take cues from how we protect ourselves in the digital environment.  A computer virus can cripple hardware and subvert software, preventing us from working or communicating. To avoid this, we have at least three options. First, we can go to the source of the virus and try to stop its spread; second, we can erect digital firewalls and install virus-detection software to halt its arrival; and third, we can try to prevent infection by distancing ourselves from it and not opening attachments. While such efforts rarely ensure complete protection, they do reduce the likelihood of a virus crippling your computer. And they suggest techniques for how we might design our cities to respond to viral diseases let loose around the globe.
Going to the Source
The first line of defense is the source of a pandemic. Viruses can mutate and spread almost anywhere, but some environments are more vulnerable. The rapid rise of overcrowded cities and so-called informal settlements in impoverished regions has produced unhealthy living conditions that that can generate and spread deadly zoonoses: diseases that transfer from animals to humans in close proximity.  With fast-growing slums now accommodating over one billion people — a number expected to double in the next three decades — the chance of an outbreak of zoonotic disease has gone up rapidly. 
Nor can we assume that the development and spread of disease will necessarily be unintentional. Slums can breed not only disease but also despair, and some observers worry that this will provoke bio-terrorism. In Planet of Slums Mike Davis argues that the “the ‘feral, failed cities’ of the Third World —especially their slum outskirts — will be the distinctive battle space of the twenty-first century. Pentagon doctrine is being reshaped accordingly to support a low-intensity world war of unlimited duration against criminalized segments of the urban poor. This is the true ‘clash of civilizations.’” 
Seattle, December 1918: Policemen wearing masks made by the Red Cross, during the influenza epidemic. [Courtesy of the National Archives at College Park, MD]
But whether for humanitarian, public health, or geopolitical reasons, the need to improve the urban conditions that give rise to viral outbreaks is urgent. The challenge — helping the billion people who live in poor shelter and unsanitary districts — might seem overwhelming. But it might be the wisest, and most cost-effective, investment we will make. The U.S. Centers for Disease Control and Prevention estimate that the economic impact of a pandemic could range from $71 billion to $166 billion, excluding the cost of disruptions to commerce and society.  Compare that to the $16 billion lent by the World Bank for shelter improvement in 90 countries over the last 34 years, and it's clear that the level of investment has come nowhere close to acknowledging the risk involved. 
Limiting the Spread
But even if the developed nations start right now to commit more money to ameliorating global poverty, progress will be slow, given the scale and diversity of needs. Which leads to the second strategy: building firewalls and detection methods to prevent viruses from spreading. The place to start is at international airports. Airline travel — which can speedily send a virus for which we have no defense around the world and cause millions of people to become infected before we even know what's happened — has unintentionally become the most efficient means of transferring disease ever invented. Researchers studying the H1N1 pandemic have detected a high correlation between the disease's spread and airline flights from Mexico in March and April of 2009. As their study states, “International air travelers departing from Mexico were unknowingly transporting a novel influenza A (H1N1) virus to cities around the world.”  Which points to the fact that while some associate contagious diseases with isolated or impoverished places, viruses become pandemic first in the most economically active and globally connected cities, which have the busiest airports and the most flights to and from the greatest number of other cities. 
Cities have a long history of fighting disease at ports and other points of entry. In the 15th and 16th centuries, when Venice suffered a series of outbreaks of bubonic plague, the government quarantined sailors and cargo for 40 days on Lazzaretto Vecchio, a small island about two miles from the Piazza San Marco.  In the 19th and early 20th centuries, New York City quarantined boat passengers on Hoffman and Swinburne Islands to halt the spread of cholera and other illnesses.  And during the 1918 pandemic, some U.S. cities required health certificates from visitors before being allowed to enter.  In the age of global air travel, efforts to slow the spread of disease through quarantine seem inconceivable. But this process has already begun, as many experienced during the flu outbreak. The U.S. Government warned arriving passengers that “their travel may be delayed” and that they might have to “ pass through a scanning device that checks your temperature” and “be quarantined for a period of time if a passenger on your flight is found to have symptoms of H1N1 flu." 
New York Harbor, Swinburne Island hospital, September 6, 1879. [Drawing: Harper's Weekly]
The risk of disease transmission may eventually change not only how we travel, but also how we work and relate globally. The old adage “think globally, act locally” may be more pertinent than ever since mobile video technology has matured to the point where we can communicate seamlessly in real time with almost anyone, anywhere. And as we become more knowledgeable about the larger, environmental costs of air travel, global commuting might seem increasingly irresponsible and wasteful. Global citizenship might mean being less physically but more electronically interconnected.
In the digital environment we try to evade viruses by avoiding the unfamiliar; in the physical environment we try to evade viruses by practicing what public health physicians call "social distancing" — another form of avoiding the unfamiliar. “In the event of a serious pandemic,” write Jeanne Lenzer and Shannon Brownlee in The Atlantic, school closings and voluntary and even mandatory quarantines "will require ... widespread buy-in from the public. Yet little discussion has appeared in the press to help people understand the measures they can take to best protect themselves." 
This has architectural and urban implications. Social distancing can take extreme and even inhuman form, as Daniel Defoe recounted in A Journal of the Plague Year; Defoe's novel took place during the Great Plague of 1665, when British officials imprisoned whole households in their own homes if even one family member showed signs of disease. But as we saw during the SARS outbreak, and this year in reaction to H1N1, people do alter behavior, changing how they relate to others and how they use public space: wearing masks, installing hand sanitizers, avoiding contact, shunning strangers, staying indoors, and even fleeing cities. 
San Francisco Naval Training Station, ca. 1945, Scene in "D" Ward of the Station Hospital, showing sneeze screens erected around beds. [Courtesy the U.S. Naval History and Heritage Command]
Pandemics can have unexpected benefits. During the plague year of 1665, when Cambridge University was temporarily closed, Isaac Newton left school, where he'd been a lackluster student, and retreated to the rural isolation of his family home in Woolsthorpe, where he proceeded to make key discoveries regarding gravity, planetary motion, optics and calculus.  But most of us will need more personal interconnection and institutional support to remain productive in the event of the social distancing that an infectious outbreak might demand. Telecommuting is an option — and in fact nearly one-quarter of the American workforce already regularly performs some part of their job from home and 62 percent wish they could do so. 
With telecommuting comes the need for more mixed-use neighborhoods that can provide a range of services to people working close to home. This might seem to counter the tactic of social distancing, but it reminds us that in earlier eras, before global travel became an easy option, most people lived in comparatively smaller and more stable communities with those who shared exposure and immunity to the same diseases. This made it essential that most daily needs be provided for within a relatively small geographic area — which also served to limit our interactions mainly to those with whom we had diseases in common. In this sense, membership in a community offered more than a social and economic benefit; it was literally a matter of life and death, since traveling too far away from one’s own viral community made a person both a threat to others and vulnerable to infection.
The prospect of pandemic, then, should spur us to rethink one of the prevailing divides in urban design — the divide between those who envision a high-tech metropolis of global connectedness, on the one hand, and those who call for a return to traditional, small-scale, mixed-use settlements, on the other. We will actually need both the high-tech metropolis and the small-scale settlement. The digital environment will globally connect us while the mixed-use settlement will provide us with the diverse local goods and services we will need in a less mobile future.
Singapore, Arriving passengers at Changi International Airport being scanned for fever in response to swine flu. [Courtesy of Jnpet]
There is also much we can do within communities and in individual buildings to protect people from viral infection. This might include rethinking the smallest details of urban life, such as the innocent, but potentially hazardous gestures of shaking hands or passing paper between two people. And it may involve our paying more attention to those elements of buildings — door knobs, light switches, restroom faucets, and the like — that we now know are points of contact for transmitting disease. Here, too, both old customs and high tech are relevant. Hand shaking used to be a sign of solidarity among members of the same (viral) community — something we would do well to remember when we greet strangers. And motion detection and remote sensing technology — now often viewed as a convenience — may become necessities as we seek ways of operating the designed environment without coming in physical contact with it.
The Viral Meme
Approaching architecture and urban design with pandemics in mind will make our cities healthier and more sustainable. Moving bits rather than bodies, increasing the mix of uses in local communities, and reducing the degree of contact we have in the physical environment — all this will not only increase our chances of surviving a pandemic, but also decrease our ecological footprint. These are related phenomena. One of the great worries in the epidemiological community is that infectious diseases will arise because of climate change, and so the more we can do to stem the latter, the more we will do to prevent the former. 
This gives new meaning to Marshall McLuhan’s idea of the "global village."  McLuhan envisioned electronic media as making the globe more like a village, with the whole planet able to communicate and connect as easily as a local community. What McLuhan couldn't see, half a century ago, but what seems likely in our virally vulnerable world, is that we may have to live, literally, in more village-like ways; we may all inhabit a digitally connected globe and, at the same time, live physically separated in our own viral communities.
That prospect will surely raise objections — it goes against the expectation of freedom that has come to characterize modern life, especially in highly developed nations. Won’t a globe of physically isolated village-cities return us to a more "primitive" existence, haunted by the tribal conflicts, ethnic prejudices, and the fear of strangers? Won’t barriers to travel impose unacceptable restrictions on our ability to experience other places and understand other cultures, and won't it constrain our ability to grow intellectually and socially beyond the bounds of our home communities?
These are precisely the questions we need to raise; it makes no sense to protect our physical health if political repression and social oppression are the results. To help prevent such unintended consequences, we might take advantage of another type of virus: the cultural virus that the biologist Richard Dawkins, in The Selfish Gene, first called a “meme” — a compelling idea that can replicate in the minds of others, increasing the intellectual diversity of a community, while also “killing off” an existing concept or unexamined assumption. 
Chicago, 1918, poster warning citizens of the dangers of the Spanish Flu Pandemic. [Courtesy of the History of Medicine (NLM)]
One challenge in fighting actual viruses is that we cannot see them, and so we downplay their importance. So it becomes especially important to make viruses somehow more visible. In this process the meme of “viral cities” might play a key role. We can transform our cities in large and small ways to protect us from pandemics — and such transformations might have the added advantage of making the threat real, making it immediate and tangible in ways that abstract public pronouncements or warnings simply cannot.
This gets to the heart of a debate among those who study memes, “memeticists” as the cognitive scientist Douglas Hofstadter called them.  Memetic “internalists” focus on the cognitive aspects of ideas, paying little attention to the physical environments that give rise to ideas, while the “externalists” argue that physical contexts can both encourage and reflect changes in our thinking. From my reading of this literature, the internalists appear to have won the debate, with neuroscience and cognitive psychology dominating the field. But when it comes to applying the meme of biological viruses as a significant shaper of human life in the 21st century, the externalists may have the edge. It may be that the physical environment — even with all the electronic gadgetry that can help us discover and transfer knowledge — will once again become fundamental to how we conceptualize and respond to the real viral threat. Only through changes in our actual behavior — in our travel expectations, work habits, living patterns, social customs, and self conceptions — will we be able to slow a pandemic long enough to develop vaccines and strengthen immunity.
This puts us in a race of sorts: which virus will embed itself? Will it be the conceptual meme of “viral cities” that spurs us to rethink our daily activities and our responsibilities to others? Or will it be the virulent gene in some actual virus for which we have no cure that arrives stealthily via an infected airline passenger and that spreads faster than we can react? Let us hope it is the former and not the latter, although the history of cities suggests otherwise. History suggests that we rarely act against an invisible threat until we have suffered the pain of ignoring it, as happened in Venice in the 14th century or London in the 17th century and again in the 19th. But maybe this time, we will be different. If we take the current threat seriously and do not dismiss it as too remote a possibility for worry, we may beat the odds and avoid the worst of the pandemics headed our way. What bets would you place on it?