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The millions of invaders are borne in by air or water. Judged by the ordinary signs of life—growth, motion, the need for food—they are dead. One by one, they find individual targets, which may be a thousand times bigger than their sub-microscopic selves. In such an unequal contest, they should have little chance. But these invaders are superlative saboteurs; they are virus particles, and their targets are cells in the human body.
Alongside the cell it is about to attack, the invader acts like an enemy agent skulking outside a big factory. Suddenly the virus slips into the complex living cell which, factory-like, has a definite schedule for receiving raw materials and processing them for the benefit of the whole organism. Either before or just after it slips in, the virus sheds its coat, a swatch of protein.
The coatless inner part of the virus is essentially a molecule of nucleic acid, but inside the factory it behaves like an immensely efficient agent taking over a captured industry. It seems to know where the production schedules and blueprints are—and it throws them away. In their place, it issues orders for the production of nothing but hundreds or thousands of copies of itself, plus an equal number of protein coats to fit. The cell-factory rushes to fill the massive order. It becomes strewn with waste materials. The strain tells. About the time the cell fills the invader's order and completes a myriad new molecules of nucleic acid, it falls into irreparable ruin. And just in time, the new molecules pick up their freshly tailored protein coats, leave the cell and go off in search of new cell-factories to conquer.
When a human victim of viral infection gets a stuffy head and a sore throat, or suffers a splitting headache and the feeling that his bones are breaking, or develops the blisters of cold sores or the rash of measles, his body is reacting to the biochemical disturbances that come from invasion by viruses. Viruses kill millions of people around the world every year, and give the miseries to hundreds of millions more.
Upstaging the Bacteria. Until only 20 years ago, the disease makers that dominated medicine's attention were the bacteria, hulking big microbes (by comparison with viruses) that generally attack by producing systemic poisons rather than by invading the body's cells. Antibiotics have wiped out or brought under control virtually all the major bacterial diseases: tuberculosis, some forms of pneumonia, diphtheria, scarlet fever, typhoid fever, gonorrhea, syphilis and most of the other illnesses that stir memories of Paul de Kruif's heroic Microbe Hunters.
But drugs cannot affect true viruses. The effect is to leave the virus diseases supreme, and virologists are multiplying with a speed reminiscent of the particles they study. Having only in the last dozen years dug out the mechanics of viral cell invasion, researchers now know of fascinating variations in the process. Occasionally and inexplicably; they have learned, viral nucleic-acid particles enter cells and "go underground," lying there dormant or masked for years. Or the nascent "pro-virus" molecules of new nucleic acid may play possum in this way. Virologists are having to coin words such as virion, capsid and capsomere to describe viral particles and their parts.