IN THE MID-I9TH CENTURY, A GERMAN PATHOLO-gist named Rudolf Virchow discovered that leukemia was caused by the rapid multiplication of abnormal white blood cells. Just like that, with some autopsy samples and a light microscope, Virchow defined cancer—a process in which healthy cells mutate and then reproduce. Before this revelation and for many decades after it, cancerous tumors were found and cut out only when they became visible or palpable. But Virchow's notion that cancer cells start out normal and then go rogue laid a foundation for modern medicine's approach to the disease: early detection. These days, we no longer have to wait for tumors to make themselves evident. We don't even have to wait for symptoms. Now doctors look for abnormal cells in healthy people, hoping to catch and remove them before they cause sickness, a strategy that has had remarkable results. Along with treatment advances, mammogra-phy has reduced the U.S. breast-cancer mortality rate by some 30% since 1989. Pap smears have helped lower the cervical-cancer mortality rate by 60% since 1975. The rate of death from colorectal cancer is also steadily dropping, thanks largely to screening.
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