In the ur-time of mythology, mans most fearsome foes were large and toothy beasts monsters and dragons, tigers, bears and sharks huge, meaty things with visible fangs and long shadows that inspired awe as much as fear. Today, most of the megafauna are dead or behind bars, more a source of nostalgia than angst. It is they who need protecting from us. We, on the other hand, must contend with a different kind of monster: microscopic creatures devoid of teeth yet able to inflict death on a horrifying scale. The latest of these minuscule terrors is, of course, the coronavirus responsible for Severe Acute Respiratory Syndrome (SARS), a pathogen that has come as close as any force in modern history to bringing mighty China to its knees.
At the time of this writing, 461 people have died from SARS; but its not the sheer numbers that count here, rather the virulence of the pest. To date, around one in 15 of those infected have died, but it now seems likely the death toll may eventually rise to 10 percent much higher than the initial estimate of 4 percent. As yet there is no reliable diagnostic test for the SARS virus and no vaccine. The only defensive tool is quarantine, whether of people, cities or entire nations.
People have always been threatened by bacterial and viral pathogens, but unlike, say, lions and tigers, for whom rising human populations are a sure precursor to annihilation, microscopic pests thrive when we proliferate. The more humans there are and the more we network with one another, the greater the chance for the microbes to multiply and mutate. As the rapid spread of SARS testifies, humans are not the only ones clocking up frequent-flier miles. Some scientists believe that within a year, two at most, the virus will be all over the globe.
The speed of SARS transmission is no surprise to Paul Ewald, an evolutionary biologist at the University of Louisville and the author of Plague Time, a sobering chronicle about the massive, modern incidence of pathogen-induced diseases. Ewald is the founder of the controversial field known as evolutionary medicine, which suggests new ways for understanding the development of pathogenic organisms and new methods for countering them.
For the past 200 years, Ewald notes, our strategy for fighting microbial invaders has been a form of biological warfare: The pathogen attacks with chemical weapons that disable the hosts cells, leading to disease and potentially death; doctors respond with counterattacks of vaccines and drugs lethal to the pathogens. For decades we may hold the beasts at bay, but behind the scenes evolution is continually at work, and sooner or later mutations arise that grant the invaders immunity. New drugs must be developed. An arms race ensues, with pathogens and researchers locked in an escalating battle to keep ahead of the others chemical innovations.
Ewald believes this arms-race approach is one that neither side can win. Rather than accelerating pathogen development, he suggests a radically different strategy in which humans take control and direct their evolution toward more benign forms. As Ewald sees it, our aim should be to domesticate pathogens, to take these wild and dangerous creatures and turn them into mild versions of what they were. Rather than trying to wipe them out, why not simply defang them? Ewald makes an analogy with the way that humans in the past responded to wolves: Once a frightening nemesis, wolves were gradually domesticated and turned into dogs. Its unlikely that a coronavirus will ever be mans best friend, but Ewald insists there is no reason why lethal pathogens cannot be transmuted into tolerable pests.
Nature reveals some interesting examples of just such a transformation. Consider the case of cholera. In 1991, the disease landed on the shores of South and Central America, striking first in Peru, then spreading quickly through the region. Some countries had properly treated water, while in others tainted water was allowed to pollute drinking supplies. Ewald notes that in the countries with proper water treatment the virulence of invading organisms dropped rapidly, and they evolved into milder strains. In countries such a Ecuador, where water treatment was poor, the pathogens evolved into even deadlier strains, and continue to remain a major problem.
We live with mild pathogens all the time. The rhinovirus responsible for the common cold affects tens of millions of people each year, and while a cold is certainly an inconvenience, its not going to kill you. Ewald imagines that with careful management we ought to be able to turn a lot more pathogens into benign, rhinolike strains. People tend to think about evolution as a source of problems, he says, noting in particular the evolution of antibiotic resistance. But Ewald sees it as a source of solutions. Though we generally imagine the evolutionary process as taking place over thousands, if not millions, of years, it can occur extremely quickly. Antibiotic resistance may develop in just a few weeks if the use of the antibiotic is sufficiently widespread.
One way to direct an organisms evolution is to control its means of transmission. In general, the harder it is for an organism to jump from one host to another, the less virulent it is forced to become. In the case of waterborne diseases like cholera, that means proper treatment of effluent. Insects such as fleas and mosquitoes are often another means of transmission, what epidemiologists call a vector. Controlling a vector-driven disease like malaria is largely a matter of combating the spread of the vector itself. In the developed world where we have controlled mosquitoes and can usually rely on our water supply, malaria, cholera, typhoid and dysentery (the latter two bacterial waterborne diseases) have been almost entirely eradicated from our lives. Yet, ironically, with the airplane we have introduced a new form of airborne vector that is proving almost as lethal as the mosquito. Not just SARS but also AIDS has been propagated around the world by this ruthlessly efficient mechanism.
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The good news is that Ewald believes the data shows that the SARS virus is not as easily transmissable as feared, and while there will continue to be hotspots, we will not be seeing a repeat of the 1918 flu pandemic.
Whatever happens, evolution will continue its onward march. Genes will recombine in unexpected ways, producing novel variants on old pathogenic themes: New varieties of colds, flus, poxes, coronaviruses, immunodeficiency viruses and things yet undreamed of inevitably await us. Like HIV, coronaviruses mutate at a very high rate and are found throughout the animal kingdom. A coronavirus vaccine widely used to immunize chickens against a form of bronchitis has recently combined with other strains to form a lethal new variety: Denied access to the birds lungs, the virus now attacks the kidneys.
Ewald believes that such a shift is unlikely to occur with human coronaviruses, but vigilance must be our catchcry. We can be absolutely certain that viruses are mutating all around us, he says. At least once a century we should expect to see a major new disease on the order of AIDS or smallpox. Its these big ones that we need to make sure we recognize early and stop before they get a firm toehold in the human population. Evolution can be our ally, he says; seen from the right viewpoint, it is a power we can harness.
Unfortunately, Ewalds approach is not likely to win him friends in the pharmaceutical industry. Unlike drugs, cultural control techniques cannot be patented, and to date his ideas have remained on the margins of public-health policy. But we are beginning to see the start of a paradigm shift and he is confident that an evolutionary perspective will eventually revolutionize our response to disease.