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