In late 2002, in the equatorial rain forest of the Republic of Congo, around a place called Mbomo, Ebola virus broke out, killing 128 people. As soon as the virus was identified, doctors and local people began taking precautions, and the outbreak stopped. The gorillas of Mbomo were not so lucky: Ebola basically wiped them out. In recent years Ebola has been sweeping through the western lowland gorilla population, apparently almost exterminating this animal in some parts of central Africa.
But the gorillas didn't start the Ebola outbreak in Congo; they were the victims as much as humans. Ebola lives quietly and naturally in some animal host in equatorial Africa, perhaps in a type of fruit bat, though nobody knows exactly. Occasionally it moves out of its natural host, spilling into humans and gorillas alike.
This was an example of a "zoonosis," as David Quammen explains in his ambitious and encyclopedic voyage into the world of emerging infectious diseases, "Spillover: Animal Infections and the Next Human Pandemic."
"When a pathogen leaps from some nonhuman animal into a person, and succeeds there in establishing itself as an infectious presence, sometimes causing illness or death, the result is a zoonosis," Mr. Quammen writes, and he tells us that the term is "a word of the future, destined for heavy use in the twenty-first century."
"Spillover" takes us on a grand tour of the world of emerging viruses. We visit Australia to learn about the 1994 outbreak of Hendra virus, Congo in search of Ebola, then on to Romania and Borneo in pursuit of malaria. We explore a cave in southern China for SARS virus, we meet Q-fever researchers in the Netherlands, we touch upon Lyme disease in New York and go on a deep voyage into the origin of AIDS in the rain forests of west-central Africa.
Mr. Quammen introduces us to scores of characters—virologists, microbiologists, epidemiologists, people who have gotten sick with eerie diseases.
Along the way, he makes it clear that the threat of future pandemics comes largely from viruses leaving the animal kingdom and finding a beautiful new home in us. Such "spillovers" are happening constantly, and they seem to be the main source of new infectious diseases in people. Influenza, SARS (severe acute respiratory syndrome) and HIV, as well as less-known but creepy pathogens such as Nipah and hantavirus (which has recently killed several people who had visited Yosemite National Park), all exist in animals, and have spilled, or are spilling, into humanity.
The reason is changing ecosystems on the planet and the growth of the human species. Many animals are facing extinction pressure or are themselves moving into new territory. Viruses that live quietly in animals but are constantly probing for new hosts are coming into more frequent contact with humans.
Every so often, a virus will jump out of an animal and infect one person, somewhere. It may then mutate, becoming able to live forever afterward in people and move efficiently from one person to the next. If a lethal virus got going this way, would doctors be able to stop it?
Mr. Quammen's account of the SARS outbreak in 2003 is particularly chilling. With precise and inquisitive reporting, he follows the virus's migration from harmless little horseshoe bats, which live in caves in southern China. The virus somehow jumped from them into civet cats in the "wet markets" of Guangdong, where wild animals are sold for food. The virus then jumped into a few people around Guangdong who had been exposed to live civets in cages.
In humans the virus was airborne, contagious and lethal. It soon got into a grandmother who flew from Hong Kong to Toronto, "taking SARS global," in Mr. Quammen's words.