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During the last 1,000 years, a series of monumental discoveries revolutionized the practice of medicine. These discoveries have saved millions of lives and brought about remarkable improvements in the health of entire populations.

One thousand years ago the average person in Europe could expect to live about 30 years. Of 100 children born alive, 40 would die before their first birthday. Disease and infections, largely the result of squalid living conditions, claimed untold numbers of lives. The average life span throughout the industrialized West has now more than doubled the average in 1000—to about 76.5 years for babies born in 1997 in the United States. Infant mortality rates are now a mere fraction of what they were 1,000 years ago.

Medical breakthroughs during the next millennium will probably bring even longer, healthier lives. Advances in genetics, for example, offer hope of new treatments to cure serious diseases such as cancer, eliminate genetic defects from families, and possibly even slow the aging process. And recent developments in transplantation technology suggest that soon it may be possible to grow replacement organs in the laboratory. But none of these anticipated breakthroughs would be possible—even thinkable—without the pioneering medical discoveries of the last 1,000 years.

Sorting through literally thousands of medical discoveries made during the past millennium to determine the ten most important is a challenge. How is the significance of a discovery measured? One factor is paramount: The most fundamental breakthroughs led to multiple other discoveries that eventually reshaped medicine and affected millions, even billions, of people. To be sure, changes in medical practices often lagged far behind the initial discovery; effective therapies sometimes emerged only after many years. These discoveries have attained the status of millennial importance not because they quickly helped physicians save lives, but because they fundamentally shifted the way scientists and physicians thought about human health. In so doing, the discoveries opened vast new fields of research that would revolutionize medicine and save the lives of incalculable numbers of people.

These seminal breakthroughs are, in the order of their discovery, human anatomy, circulation of blood, bacteria, vaccination, surgical anesthesia, X rays, blood typing, tissue culture, antibiotics, and the structure of deoxyribonucleic acid (DNA).

Setting the Stage

The practice of medicine is as old as civilization itself, but the Greeks are generally credited with inventing the science of medicine—using observation and experience rather than appeals to supernatural forces to treat disease. Although Greek medical knowledge was passed on to the conquering Romans, it fell into obscurity as the Roman Empire collapsed in the early Middle Ages.

A period of stagnation in the sciences, combined with sporadic epidemics of the bubonic plague, smallpox, and other diseases, reinforced the turn toward superstition and magical treatments in medieval Europe. Only fragments of the ancient medical learning survived. Many people viewed disease as a form of punishment for sins or as the result of demonic forces. Prayer was a standard form of treatment.

Western medicine received a major boost when the Italian universities of Salerno, Bologna, and Padua established medical faculties in the 9th and 10th centuries. By the 12th century, the University of Paris in France and Oxford University in England had also founded faculties of medicine. These institutions provided facilities for research, set examination requirements for graduating physicians, and laid the foundations for the extraordinary revival of Western medicine in the 16th and 17th centuries—a revival that has continued to this day.

Human Anatomy

Before modern medical science could emerge, medical practitioners needed an accurate understanding of human anatomy. Without clear descriptions of the structure of the human body, it was impossible to learn what different bodily parts actually do. Once researchers understood how parts of the body worked, they were better able to devise medical therapies to restore proper functions.

Amazingly, no one knew much about human anatomy until 1543, when the Belgian anatomist Andreas Vesalius wrote De Humani Corporis Fabrica, Libri Septem (On the Structure of the Human Body, in Seven Books). For centuries, exploration of the anatomy of human corpses was forbidden. In medieval Europe, knowledge about anatomy was based largely on the teachings of the Roman physician Galen (129-199?). Galen's anatomical descriptions were based on dissections of animals, which differ in many ways from humans. But contradicting Galen was dangerous because the powerful Roman Catholic Church accepted his findings as gospel. A few brave souls had tried to correct some of Galen's errors, but their work was lost for centuries.

Ambitious, driven, and ruthless, 23-year-old Vesalius received his medical degree in 1537 from the University of Padua and was immediately appointed head of surgery and anatomy there. As a student, and later as a scientist, he recovered human corpses from cemeteries late at night. He even encouraged his students to note patients who were at death's door so that he could steal their bodies for dissection before they were buried. Vesalius slept, night after night, with corpses in his own bedroom, and he hired Italy's greatest artists to draw what he found.

In 1543 Vesalius completed his seven-book masterpiece, richly illustrated with more than 200 magnificent drawings. Many consider it one of the greatest medical books ever published. This monumental work gave medicine a precious gift: For the first time, human anatomy was based on careful dissection and observation rather than on a rigid orthodoxy rooted in ancient texts.

Circulation of Blood

English physician William Harvey's discovery of what the heart does and how the blood circulates is widely regarded as the single greatest medical achievement of all time: It established the principle of doing experiments in medicine to learn how the body's organs and tissues function. Published in 1628, Harvey's groundbreaking book Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (Anatomical Essay on the Motion of the Heart and Blood in Animals) spurred research into the mechanical functions of many bodily processes, including respiration, digestion, metabolism, and reproduction.

Harvey received his medical degree from the University of Padua, where he learned one very important fact: Veins have valves that permit blood to travel in only one direction. However, the exact role of the valves was unclear.

Realizing that it was still dangerous to contradict Galen, who had claimed that the liver not only makes the body's blood but also pumps it through the body, Harvey decided to study blood flow by operating on live animals. For a period of 12 years Harvey conducted his experiments before members of the influential Royal College of Physicians in London, England. He wanted their support for his book, which praised Galen while challenging many of his ideas.

In the 8th chapter of his 17-chapter book, Harvey carefully introduced the revolutionary idea that blood goes in a circle in the body, traveling from the heart to the arteries to the veins and back to the heart. The next 9 chapters proved, in wonderfully clear English, that he was right.

In a series of brilliant experiments in animals and humans, Harvey demonstrated how blood circulates in the body. When an artery was blocked, the veins draining this artery collapsed. When a vein was blocked, it swelled below the blockage and collapsed above it, but the swelling disappeared when the blockage was removed. He also showed that the valves in veins allow blood to flow only in the direction of the heart. Together, these discoveries proved that blood moves in a circle in the body—that is, there is a “circulation.”

Bacteria

After the momentous medical breakthroughs of Vesalius and Harvey came the 17th-century discovery of one of the human body's greatest enemies: bacteria. This discovery eventually led to the realization that exposure to certain microorganisms could cause disease. It also prompted new theories of antiseptics that sharply lowered mortality rates from surgery.

Antoni van Leeuwenhoek, a part-time janitor and haberdasher working in Delft, Holland, discovered bacteria and other microorganisms using a microscope that he built himself. Through the influence of a friend, a Dutch physician, he was invited to write letters to the Royal Society of London—a group dedicated to the advancement of science. These letters were translated from Dutch into English and published in the society's journal Philosophical Transactions.

Leeuwenhoek's most famous letter was published on March 16, 1677. In this letter he described looking at a drop of rainwater through his microscope. The drop was taken from a tub where it had been allowed to stand for several days. To his amazement, he saw exceedingly tiny animals, known today as protozoa, swimming in the water. He also observed other equally small animals that did not move at all, now known as bacteria. No one at the Royal Society knew anything about these little creatures, which Leeuwenhoek called animalcules. At the request of the stunned members of the Royal Society, several of the most respected citizens of Delft were asked to verify Leeuwenhoek's microscopic findings. They did so, and in 1680 Leeuwenhoek was elected a fellow of the prestigious Royal Society.

Later discoveries extended