MEDICINE: The Killers All Around

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Because microbial infections keep finding ways to outsmart antibiotics, doctors are convinced that vaccines are a better way to combat bacterial disease. A vaccine is usually made from a harmless fragment of microbe that trains the body's immune system to recognize and fight the real thing. Each person's immune system is chemically different from everyone else's, so it's very difficult for a bacterium to develop a shield that offers universal protection. Diphtheria and tetanus can be prevented by vaccines if they are used properly. A vaccine against the pneumococcus bacterium has recently come out of the lab as well, and scientists expect to test one that targets streptococcus A within a year.

VIRUSES

Unlike bacteria and protozoans, which are full-fledged living cells, capable of taking in nourishment and reproducing on their own, viruses are only half alive at best. They consist of little more than a shell of protein and a bit of genetic material (DNA or its chemical cousin RNA), which contains instructions for making more viruses -- but no machinery to do the job. In order to reproduce, a virus has to invade a cell, co-opting the cell's own DNA to create a virus factory. The cell -- in an animal, a plant or even a bacterium -- can be physically destroyed by the viruses it is now helplessly producing. Or it may die as the accumulation of viruses interferes with its ability to take in food.

It is by killing individual cells in the body's all-important immune system that the AIDS virus wreaks its terrible havoc. The virus itself isn't deadly, but it leaves the body defenseless against all sorts of diseases that are. Other viruses, like Ebola, kill immune cells too, but very quickly; the dead cells form massive, deadly blood clots. Still others, hantavirus, for example, trigger a powerful reaction in which immune cells attack both the invading virus and the host's healthy cells.

Unlike bacteria and protozoans, viruses are tough to fight once an infection starts. Most things that will kill a virus will also harm its host cells; thus there are only a few antiviral drugs in existence. Medicine's great weapon against viruses has always been the preventive vaccine. Starting with smallpox in the late 1700s, diseases including rabies, polio, measles and influenza were all tamed by immunization. $

But new viruses keep arising to challenge the vaccine makers. They may have gone undetected for centuries, inhabiting animal populations that have no contact with mankind. If people eventually encounter the animals -- by settling a new part of the rain forest, for example -- the virus can have the opportunity to infect a different sort of host.

Scientists believe Ebola virus made just that kind of jump, from monkeys into humans; so did other African viruses such as Marburg and the mysterious X that broke out in Sudan. And many more are likely to emerge. "In the Brazilian rain forest," says Dr. Robert Shope, a Yale epidemiologist, "we know of at least 50 different viruses that have the capacity of making people sick. There are probably hundreds more that we haven't found yet."