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even if they can prolong lives, are not the perfect cure. They have side effects that can become harmful after years of therapy, and they foster the evolution of resistant viruses, which then requires shifting patients to new drugs. In theory, the best solution to HIV would be a vaccine—either one that could prevent people from becoming infected with the virus or one that could stimulate the immune system of infected people to attack it effectively. Vaccines would be far less expensive than treating HIV infection with drug cocktails and could help slow down the transmission cycle. But the quest for an HIV vaccine has been a disappointing struggle so far. In 2008, for example, a highly anticipated trial of a vaccine developed by Merck had to be abandoned because the vaccine appeared to be making people more likely to acquire HIV, not less.

There’s good reason to worry about any HIV vaccine, even one that shows promise in small trials. That’s because HIV is evolving in overdrive. HIV belongs to a group of viruses—including influenza—that are very sloppy in their replication. They create many mutants in very little time. These mutants provide the raw material for natural selection to act on, producing viruses that are better and better adapted. Within a single host, natural selection can improve the ability of viruses to escape detection of the immune system.

In 2008, Philip Goulder, a medical researcher at Oxford, led an international team of scientists who found evidence for the ongoing evolution of HIV. They studied the immune systems of 2,800 people from all over the world, examining proteins known as human leukocyte antigens, which infected cells use to transport fragments of viruses to their surface. The fragments can then be recognized by immune cells, which destroys the infected cell. Different people carry different variations in the genes for human leukocyte antigens. Goulder and his colleagues found that most of the HIV in each country carried mutations to the most effective human leukocyte antigens in that country’s population. Their findings tell us that HIV is rapidly adapting to the variations in human immune systems around the world. That is sobering news to those who are trying to build HIV vaccines. If a vaccine ever succeeds in boosting an effective immune response in people, HIV might well evolve a way to escape.

It’s possible that vaccine developers could keep HIV from escaping by continually rolling out new vaccines that would stay one step ahead of the virus. Another intriguing possibility is to look back over its history. A team of American scientists compared a wide range of HIV-1 subtype B strains and reconstructed one of the proteins made by their common ancestor. They then used that ancestral protein to make a vaccine. The researchers found that monkeys injected with the vaccine were able to produce an immune response to a much wider range of HIV strains than more conventional vaccines. The future of fighting HIV, perhaps, may lie in its past.

Becoming an American

West Nile Virus

In the summer of 1999, Tracey McNamara got worried. McNamara was the chief pathologist at the Bronx Zoo. When an animal at the zoo died, it was her job to figure out what killed it. She began to see dead crows on the ground near the zoo, and she wondered if they were being killed by some new virus spreading through the city. If the crows were dying, the zoo’s animals might start to die too.

Over Labor Day weekend, her worst fears were realized: three flamingoes died suddenly. So did a pheasant, a bald eagle, and a cormorant. McNamara examined the dead birds and found they had all suffered bleeding in their brains. Their symptoms suggested that they had been killed by the same pathogen. But McNamara could not figure out what pathogen was responsible, so she sent tissue samples to government laboratories. The government scientists ran test after test for the various pathogens that might be responsible. For weeks, the tests kept coming up negative.

Meanwhile, doctors in Queens were seeing a worrying