A Planet of Viruses - Carl Zimmer (2011)


Predicting the Next Plague

Severe Acute Respiratory Syndrome and Ebola

A hunter emerges from a tropical forest, a shotgun in one hand, the carcass of a monkey in the other. He walks into a village in the southeast corner of Cameroon. It’s a scene that replays itself every day in villages, not just in Africa, but around the world. Hunters kill wild animals and bring them home to feed their families or to sell for cash. But on this day, the scene ends with a twist. The hunter hands over the monkey to his wife to butcher. As she cuts up the monkey, she stops to hold a dismembered leg over a piece of paper marked with five circles. Drops of blood fill one circle after another. The hunter’s wife then slips the sheet of paper in a Ziploc bag and hands it to a team of scientists who have paid her a visit. The scientist, who belongs to an organization called the Global Viral Forecasting Initiative, will analyze the blood-soaked paper for viruses infecting the monkey.

The Global Viral Forecasting Initiative is trying to change the way we fight viruses. Someday, somewhere, a virus we don’t know about is going to emerge as a major new threat to human health. We’ve seen it happen many times before, and so we know it will happen again. GVFI scientists think we’ll do a better job fighting that new virus if we can learn something about it in advance. To eliminate the advantage of surprise, GVFI scientists are looking for these viruses before they jump into humans. The best place to look for them is in animals, such as the monkeys that Cameroonian hunters kill for food.

The threat of new viruses has inspired a string of cheesy Hollywood movies over the years. In The Andromeda Strain, which came out in 1971, a satellite falls to Earth with an extraterrestrial virus that threatens to wipe out humanity. In the 1995 movie Outbreak, a monkey imported from Africa spreads a deadly virus through a California town, which the Army wants to bomb to prevent it from spreading across the country. And in 28 Days Later, released in 2002, a virus sweeps through London, turning its victims into homicidal maniacs.

The reality of new viruses is nothing like these fantasies. In its own way, it’s far more frightening. Over the course of human history, many viruses have made the evolutionary leap from animal hosts to our own species. And just over the past century, dozens of viruses have made this transition, giving rise to new diseases. Scientists have found that these new viruses have generally taken the same route into our species. It’s likely that they will take the same path in the future.

Many human viruses evolved from ancestral pathogens that were well adapted to living in other species. For example, HIV evolved from a virus found in chimpanzees known as SIVcpz. For centuries, the virus moved from chimpanzee to chimpanzee, infecting immune cells and slowly eroding their defenses. In the early 1900s, some of the viruses moved from chimpanzees to humans, evolving into HIV. The most HIV-like strains of SIVcpz are carried by chimpanzees that live in the forests in Cameroon. It was there that the virus likely made the transition. Both SIVcpz and HIV are spread through blood-to-blood contact. SIVcpz probably first infected the hunters who killed chimpanzees for meat. The virus-laden blood in the butchered apes made contact with cuts on the hunters, delivering SIVcpz into new hosts.

When animal viruses first make contact with humans, they only use them as what scientists called “spillover hosts.” Adapted to growing in other animals, the viruses can only grow slowly in humans and typically fail to spread from one human to another. When SIVcpz started infecting hunters, it probably still depended on chimpanzees to replenish its numbers. But the viruses were also mutating rapidly, and mutant SIVcpz eventually evolved the ability to survive in humans and spread from one human to the next.

Initially, new human viruses only cause local outbreaks, because they still can’t move between people very well. After each human epidemic sputters to an end, the virus still thrives in its animal host. But as the virus spends more time in humans, natural selection favors mutations that adapt them to their new host. The epidemics in humans get bigger and last longer. HIV, for example, thrived as African colonies grew and networks of roads linked forest villages to large cities where the virus could circulate among many people. As HIV became better adapted to infecting humans, it lost its ability to attack chimpanzees.

No one knew about the transformation of HIV while it was happening. Only in the early 1980s, sixty years or so after the virus had crossed into our species, did scientists finally isolate the virus and realize it was causing AIDS. By then, HIV was well established in our species and started to become one of the worst diseases in human history. We can only speculate about how much easier it would have been to fight the disease back when it was infecting just a few hundred villagers in Cameroon.

In recent years, scientists have been able to identify new human diseases far faster. In November 2002, for example, a Chinese farmer came to a hospital suffering from a high fever and died soon afterward. Other people from the same region of China began to develop the disease as well, but it didn’t reach the world’s attention until an American businessman flying back from China developed a fever on a flight to Singapore. The flight stopped in Hanoi, where the businessman died. Soon, people were falling ill in countries around the world, although most of the cases turned up in China and Hong Kong. About 10 percent of people who became sick died in a matter of days. The disease was not one that any doctor had identified before—not the flu, not pneumonia, nor any other known disease. It was dubbed severe acute respiratory syndrome, or SARS.

Scientists began searching samples from SARS victims for a cause of the diseases. Malik Peiris of the University of Hong Kong led the team of researchers who found it. In a study of fifty patients with SARS, they discovered a virus growing in two of them. The virus belonged to a group of species called coronaviruses, which can cause colds and the stomach flu. Peiris and his colleagues sequenced the genetic material in the new virus and then searched for matching genes in the other patients. They found a match in forty-five of them.

Based on their experience with viruses such as HIV, scientists suspected that the SARS virus had evolved from a virus that infects animals. They began to analyze viruses in animals with which people in China have regular contact. As they discovered new viruses, they added their branches to the SARS evolutionary tree. In a matter of months, scientists had reconstructed the history of SARS.

The virus started in Chinese bats. A lineage of the viruses then began to spill over into a catlike mammal called a civet. Civets are a common sight in Chinese animal markets, and it’s likely that humans became spillover hosts as well. The virus then evolved the ability to leap from human to human. SARS was a very young virus when scientists discovered it, and the speed at which it was discovered helped make it a relatively small outbreak. Scientists were able to identify and quarantine people with the disease, and they banned the sale of civets in markets. Although the SARS virus managed to spread across much of the world, it only caused about eight thousand cases and nine hundred deaths before it disappeared.

We can expect more viruses to sweep into our species, and they will probably come at an accelerating pace. Animals in remote parts of the world have harbored exotic viruses for millions of years, and for all that time humans have had little contact with them. Now humans are moving deep into these remote territories to harvest timber, dig mines, and establish new farms. And in the process, they’ve come into contact with new viruses. Nipah virus, for example, causes dangerous inflammation of the brain in its victims in Southeast Asia. It’s a virus that normally lives in bats, which once lived far from humans in jungles. Now the bats—and the viruses—have no jungles to live in.

There’s no reason to think that one of these new viruses will wipe out the human race. That may be the impression that movies like The Andromeda Strain give, but the biology of real viruses suggests otherwise. Ebola, for example, is a horrific virus that can cause people to bleed from all their orifices, including their eyes. It can sweep from victim to victim, killing almost all its hosts along the way. And yet a typical Ebola outbreak only kills a few dozen people before coming to a halt. The virus is just too good at making people sick, and so it kills its victims faster than it can find new ones. Once an Ebola outbreak ends, the virus vanishes for years.

Ebola-like viruses may be frightening, but they probably pose less of a danger to our species than viruses with a lower death rate that can spread to more hosts. The 1918 outbreak of influenza killed only a tiny fraction of its victims. But because it infected one in three people on Earth, that tiny fraction added up to an estimated fifty million people. HIV crept slowly and surreptitiously around the planet before it was first detected. Instead of causing the terrifying symptoms of Ebola, HIV quietly breaks down the immune system over the course of many years.

We don’t know which virus will create the next great epidemic, in part because we don’t know the world of viruses very well. GVFI scientists have discovered a number of new viruses in African monkeys. Their tests on hunters have revealed those viruses in humans as well. Fortunately, these new viruses cannot yet spread from person to person. But that doesn’t mean that we can simply ignore them. Just the opposite: these are the viruses we need to block before they get a chance to make the great leap into our species.