SARS. Ebola. Avian flu. The parade of frightening new maladies continues, each one confirming that our species, for all its cleverness, still lives at the mercy of the microbe. It didn’t seem that way 30 years ago–not with smallpox largely defeated, AIDS still undreamed of and medical science evolving at an unprecedented clip. But even as optimists proclaimed victory over the germ, our megacities, factory farms, jet planes and blood banks were opening broad new avenues for infection. The dark side of progress is now unmistakable; many of the advances that have made our lives more comfortable have also made them more dangerous. Some 30 new diseases have cropped up since the mid-1970s–causing tens of millions of deaths–and forgotten scourges have resurfaced with alarming regularity. “Infectious diseases will continue to emerge,” the Institute of Medicine declares in a new report, warning that complacency and inaction could lead to a “catastrophic storm” of contagion. So what’s to be done? As the SARS outbreak has shown, surveillance is critical. By spotting new infections wherever they occur, and working globally to contain them, we can greatly reduce their impact. But is preparedness our ultimate weapon? Do we know enough about the genesis of new diseases to prevent them? Could we avert the next SARS? The next AIDS? What would a reasonable strategy look like?
We don’t hold all the cards in this game. Most new diseases begin when a person catches something from an animal–a transaction shaped by chance or even the weather. When healthy young adults started dying of a SARS-like syndrome in New Mexico 10 years ago, it took health experts several weeks of intensive lab work to identify the culprit. To the scientists’ amazement, it wasn’t a human pathogen at all. It was a novel member of the hantavirus family, a group of rodent viruses that sometimes spread through the air after rats or mice shed them in their urine. The previous outbreaks had occurred in Asia. So why were people dying in New Mexico? Scientists now believe the American mice had harbored the virus all along but had never been populous enough to scatter infectious doses in people’s toolsheds and basements. What changed the equation that year was El Nino. The ocean disturbance caused an unusually warm winter in the Southwest. The mouse population exploded as a result–and the hantavirus got a free ride.
Until someone harnesses the jet stream, such accidents are sure to happen. But quirky weather isn’t the greatest threat we face. As ecologists study the causes of disease emergence, they’re finding that human enterprise is a far more significant force. Almost any activity that disrupts a natural environment can enhance the mobility of disease-causing microbes. Consider what happened in the 1980s, when farmers in Venezuela’s Portuguesa state cleared millions of acres of forest to create cropland. The farms drew as many rats and mice as people, and the rodents introduced a deadly new virus into the region. The so-called Guanarito virus causes fever, shock and hemorrhaging. It infected more than 100 people, leaving a third of them dead.
Malaysian pig farmers had a similar experience in 1999, after they started pushing back the forest to expand their operations. As barns replaced forestland, displaced fruit bats started living in the rafters, bombarding the pigs’ drinking water with a pathogen now known as the Nipah virus. “The pigs developed an explosive cough that became known as the one-mile cough because you could hear it from so far away,” says Mary Pearl, president of the Wildlife Trust in Palisades, N.Y. The virus soon spread from the pigs to their keepers, causing extreme brain inflammation and killing 40 percent of the affected people. The outbreak ended when Malaysian authorities closed eight farms and slaughtered a million pigs.
Fortunately, you’re not likely to spread it in either place. Even when a microbe succeeds at leaping from one species to another, the new host is often a dead end. Neither Nipah nor Guanarito can spread from person to person. The hantaviruses have the same problem. And a tick could suck on a Lyme-disease patient all day without getting enough bacteria to infect its next host. The infections we get from primates and pigs are a whole different story. When the Ebola virus jumps from an ape into a person, it often races through a family or a hospital before burning itself out. And HIV is still spreading steadily after three decades of person-to-person transmission. It has infected some 60 million people since crossing over from chimpanzees, and its emergence was no fluke of the weather. We placed ourselves in the path of the virus, we moved it around the world, and we’re well poised to do it again.
The human AIDS viruses are descended from simian pathogens known as SIVs. HIV-1 is essentially a chimpanzee virus, while HIV-2 (a rarer and milder bug) comes from the sooty mangabey (a monkey). How did the chimp virus make its way into humans? The best guess is that African hunters contracted it while butchering animals, and then passed it on through sexual contact. Until a few decades ago, that hunting accident would have been a local misfortune, a curse played out in a few rural villages. What turned it into a holocaust was not just a new infectious agent but a proliferation of roads, cities and airports, a breakdown of social traditions, and the advent of blood banking and needle sharing. Those conditions virtually sealed HIV’s success, and they continue to rocket obscure bugs into every corner of the world. “The volume and speed of travel are unprecedented,” says Dr. Mary Wilson of Harvard. “We are interconnected in ways that weren’t true a century ago.”
SARS is only the latest reminder of how powerful those connections can be. The novel coronavirus that causes the syndrome emerged from Guangdong, the same Chinese province that delivers new flu viruses to the world most years. Pigs, ducks, chickens and people live cheek-by-jowl on the district’s primitive farms, exchanging flu and cold germs so rapidly that a single pig can easily incubate human and avian viruses simultaneously. The dual infections can generate hybrids that escape antibodies aimed at the originals, setting off a whole new chain of human infection. The clincher is that these farms sit just a few miles from Guangzhou, a teeming city that mixes people, animals and microbes from the countryside with travelers from around the world. You could hardly design a better system for turning small outbreaks into big ones.
For all the fear it has caused, SARS clearly isn’t the big one, at least not in its current incarnation. The coronavirus that causes it is as nasty as any flu virus, but it doesn’t get around very easily. And as University of Louisville evolutionist Paul Ewald points out, an epidemic can’t sustain itself unless each patient infects more than one other person. “If each SARS case were generating even two others,” he says, “we would have seen hundreds of thousands by now.” A doomsday flu virus would approach the virulence of the SARS agent, but it would infect people by the roomful.
Such pandemic flu viruses have emerged in the past, and many experts believe it’s only a matter of time until it happens again. How can we lessen the danger? A long-term strategy would have to include modernizing the world’s farms, improving basic health care and stockpiling vaccines and antiviral drugs. As science illuminates the ecology of infectious disease, it may also inspire wiser, safer approaches to land use and wilderness preservation. Until then, surveillance will be doubly important. The good news is that the forces making microbes so mobile are also making them easier to track. Ten years ago, quick communication was still a problem for many health departments, says Stephen Morse, director of the Center for Public Health Preparedness at Columbia University’s Mailman School of Public Health. “A colleague in Russia had a fax but no fax paper. A colleague in Ghana had telex but no fax. In other places they had a telephone but no telex.” Today even the most remote surveillance stations are tied into the Web-based Program for Monitoring Emerging Diseases. The world’s largest health agencies have created similar systems for sharing scientific research. Such systems are only as good as the openness and good will of their users. If anything good has come of the SARS scare, it is a renewed commitment to those ideals. How far they’ll take us is still anyone’s guess.
