MEDICINE: The Killers All Around

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Viruses like Ebola and X are scary, but they're too deadly to be much of a threat to the world. Their victims don't have much of a chance to infect others before dying. In contrast, HIV, the AIDS virus -- which may have come from African primates as early as the 1950s -- is a more subtle killing machine, and thus more of an evolutionary success. An infected person will typically carry HIV for years before symptoms appear. Thus, even though HIV doesn't move easily from one human to another, it has many chances to try. Since the first cases were reported in the late 1970s, HIV has spread around the world to kill perhaps a million people and infect an estimated 17 million.

It isn't just new viruses that have doctors worried. Perhaps the most ominous prospect of all is a virulent strain of influenza. Even garden-variety flu can be deadly to the very old, the very young and those with weak immune systems. But every so often, a highly lethal strain emerges -- usually from domesticated swine in Asia. Unlike hiv, flu moves through the air and is highly contagious. The last killer strain showed up in 1918 and claimed 20 million lives -- more than all the combat deaths in World War I. And that was before global air travel; the next outbreak could be even more devastating.

Vaccines should, in theory, work just as well for new varieties of disease as they do for old ones. In practice, they often don't. An HIV vaccine has proved difficult to develop because the virus is prone to rapid mutations. These don't affect its deadliness but do change its chemistry enough to keep the immune system from recognizing the pathogen.

Creating a vaccine for each strain of flu isn't exactly simple either. "First," says Yale's Shope, "we have to discover something new is happening. Then we have to find a manufacturer willing to make a vaccine. Then the experts have to meet and decide what goes into the vaccine. Then the factory has to find enough hens' eggs in which to grow the vaccine. There are just a lot of logistical concerns."

People are partly to blame for letting new viruses enter human populations. Says Dr. Peter Jahrling, senior research scientist at the U.S. Army Medical Research Institute of Infectious Diseases: "If you're a monkey imported from the Philippines, your first stop when you hit this country is a quarantine facility. If you're a free-ranging adult human being, you just go through the metal detector and you're on your way."

Sometimes environmental changes help microbes move from animals to humans. Lyme disease, a bacterial infection, was largely confined to deer and wild mice until people began converting farmland into wooded suburbs -- which provided equally good habitats for the animals and the bacteria-infested ticks they carry and also brought them into contact with large numbers of people. The mice that transmit the hantavirus often take refuge in farmers' fields, barns and even homes. Air-conditioning ducts create a perfect breeding ground for Legionnaires' disease bacteria. Irrigation ditches and piles of discarded tires are ideal nesting spots for the Aedes aegypti mosquito, carrier of dengue and yellow fevers; imported used tires have already brought the Asian tiger mosquito, also a carrier of dengue, into the U.S.