Vaccination is one of medicine's cleverest tricks: making the body believe it is sick and thus causing it to marshal just the right forces to ward off that particular sickness. The development of this practice stands as a twentieth-century accomplishment, but its roots reach far back into the past. Centuries ago, the Chinese and the Turks knew enough to produce a medicine against smallpox by grinding up the scabs of people with mild cases of the disease. In 1796, Dr. Edward Jenner found he could induce resistance to smallpox by using the vaccination virus (vacca is Latin for cow) to infect people with the relatively mild cowpox. But it was Louis Pasteur, working a century later, who did the research that finally gave the field of immunology the creative boost that would propel it to the forefront of modern medicine. In 1985, Pasteur produced a rabies vaccine without actually realizing that he was enhancing the body's own immune system; he knew only that the vaccine worked.

But what was the infectious agent that vaccine fought? Could it have been a bacterium? In Germany, in 1982, Robert Koch had shown that just such a germ caused tuberculosis. Microscopic parasites with similarities both to plants and animals, bacteria were certainly the cause of much human misery. But they were not to play the starring role in the vaccine story.

The first tantalizing awareness of virus—a micro-organism even stranger than the invisible bacteria and like nothing else ever known before—came in 1898 when Martinus Willem Beijerinick discovered a minuscule living thing he described with a name, "virus", derived from the Latin for poisonous slime. A virus is really no more than a protein bag carrying its own set of generic instructions. A virus cannot reproduce on its own. It must attach itself to a cell, impregnate the cell with viral genes, and then, parasite that it is, turn that cell into a reproductive machine for the virus's benefit. The body, for the most part, is able to recognize these viruses as foreign invaders by the signature proteins on their surface. It then attacks them with antibodies and sends killer cells to destroy the cells that have already been infected. If the immune system is overwhelmed by the invasion, the body becomes sick and may die. If the body wins, then its immune system keeps a record of this particular enemy and is better prepared to resist the next time. Sometimes the immunity is lifelong.