hid continued grow and divide. From time to time, an infected CD4 T cell wakes up and fires a blast
of viruses that infect new cells. The immune system attacks these new waves, but over time it
becomes exhausted and collapses.
It may take only a year for an immune system to fail, or more than twenty. But no matter how long it
takes, the outcome is the same: people can no longer defend themselves against diseases that would
never be able to harm a person with a healthy immune system. In the early 1980s, a wave of HIV-
infected people began to come to hospitals with strange diseases like pneumocystis pneumonia.
Doctors discovered the effects of HIV before they discovered the virus, dubbing it acquired
immunodeficiency syndrome, or AIDS. In 1983, two years after the first AIDS patients came to light,
French scientists isolated HIV from a patient with AIDS for the first time. More research firmly
established HIV as the cause of AIDS. Meanwhile, doctors were discovering more cases of AIDS,
both in the United States and abroad. Other great scourges, such as malaria and tuberculosis, are
ancient enemies, which had been killing people for thousands of years. Yet HIV went from utter
obscurity in 1980 to a global scourge in a matter of a few years. Here was an epidemiological
mystery.
To solve it, scientists began to sequence the genes of HIV they isolated from different patients.
They examined HIV not just from the United States but from other countries around the world where it
was beginning to spread as well. They drew evolutionary trees, with each strain of HIV a branch
sprouting from a common ancestor. Researchers discovered that there was not one kind of HIV, but
two. The vast majority of cases of HIV were caused by a strain that was dubbed HIV-1, and the rest
were caused by a distinct form of the virus, called HIV-2. The two types of HIV could be
distinguished in many ways, including the symptoms they caused: HIV-2 was much milder than HIV-1.
HIV, scientists found, belongs to a large group of slow-growing retroviruses, known as
lentiviruses. Lentiviruses infect many mammals, including cats, horses, cows, and monkeys. In 1991,
Preston Marx of New York University and his colleagues discovered that HIV-2 was closely related
to lentiviruses that infect an African species of monkey called sooty mangabeys. They concluded that
HIV-2 descended from a mangabey lentivirus. In West Africa, where HIV-2 is most common, some
people keep the monkeys as pets; others eat them. Infected mangabeys may have introduced their
lentivirus into humans with a bite.
It took scientists longer to pin down the origins of HIV-1, the strain that causes the vast majority of
AIDS cases. That’s because the closest relatives of HIV-1 lived in primates that are much harder to
study: chimpanzees. Relatively few chimpanzees live in captivity, and trying to get blood samples
from chimpanzees in the wild can be a staggeringly hard job. They’re elusive, strong, and not fond of
people with needles. Scientists had to develop new ways to test them for HIV, such as searching for
the viruses in their feces. Slowly, scientists amassed a collection of HIV-1–like lentiviruses from
chimpanzees. Comparing the viruses to each other, they discovered that some strains of HIV-1 are
more closely related to certain chimpanzee viruses that they are to other HIV-1 strains. The
branchings of the viral tree suggest that HIV-1 actually evolved from chimpanzee viruses several
times.
But when did this transition happen? Some scientists tried to get an answer to that question by