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.