A Planet of Viruses - Carl Zimmer [28]
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