the New World are strains of HPV found in Asia, just as Native Americans are most closely related
to Asians.
Columbus’s discovery of the New World triggered a second wave of new viruses. Europeans
brought viruses causing diseases such as influenza and smallpox that wiped out most Native
Americans. In later centuries, still more viruses arrived. HIV came to the United States in the 1970s,
and at the end of the twentieth century, West Nile virus became one of America’s newest immigrants.
It had only been six decades since West Nile virus was discovered anywhere on the planet. In
1937, a woman in the West Nile district of Uganda came to a hospital with a mysterious fever, and her
doctors isolated a new virus from her blood. Over the next few decades, scientists found the same
virus in many patients in the Near East, Asia, and Australia. But they also discovered that West Nile
virus did not depend on humans for its survival. Researchers detected the virus in many species in
birds, where it could multiply to far higher numbers.
At first it was not clear how the virus could move from human to human, from bird to bird, or from
bird to human. That mystery was solved when scientists found the virus in a very different kind of
animal: mosquitoes. When a virus-bearing mosquito bites a bird, it sticks its syringe-like mouth into
the animal’s skin. As the mosquito drinks, it squirts saliva into the wound. Along with the saliva
comes the West Nile virus.
The virus first invades cells in the bird’s skin, including immune system cells that are supposed to
defend animals from diseases. Virus-laden immune cells crawl into the lymph nodes, where they
release their passengers, leading to the infection of more immune cells. From the lymph nodes,
infected immune cells spread into the bloodstream and organs such as the spleen and kidneys. It takes
just a few days for the viruses in a mosquito bite to multiply into billions inside a bird. Despite their
huge numbers, West Nile viruses cannot escape a bird on their own. They need a vector. An mosquito
must bite the infected bird, drawing up some of its virus-laden blood. Once in the mosquito, the
viruses invade the cells of its midgut. From there they can be carried to the insect’s salivary glands,
where the viruses are ready to be injected into a new bird.
Vector-borne viruses like West Nile virus require a special versatility to complete their life cycle.
Mosquitoes and birds are profoundly different kinds of hosts, with different body temperatures,
different immune systems, and different anatomies. West Nile virus has to be able to thrive in both
environments to complete its life cycle. Vector-borne viruses also pose special challenges to doctors
and public health workers who want to stop their spread. They don’t require people to be in close
contact to spread from host to host. Mosquitoes, in effect, give the viruses wings.
Studies on the genes of West Nile virus suggest that it first evolved in Africa. As birds migrated
from Africa to other continents in the Old World, they spread the virus to new bird species. Along the
way, West Nile virus infected humans. In Eastern Europe, epidemics broke out, producing some cases
of encephalitis. In a 1996 epidemic in Romania, ninety thousand people came down with West Nile,
leading to seventeen deaths. These new epidemics, first in Europe and later in the West, may have
been the result of the virus infecting people who populations had not experienced it before. In Africa,
by contrast, people may be immunized against West Nile virus after being infected while they’re
young.