Healthy cells have regulating mechanisms that generally limit how rapidly they can divide. Skin cells, for example, normally divide about once every 30 days, but they can divide faster in response to a wound that needs healing. Cancer, however, is characterized by cell division that has gone out of control. In cancer cells, the mechanisms that regulate division break down, and the cells spend less time in the quiescent state and more time dividing.

Many chemotherapy drugs work by interfering with the cell-division cycle. The drugs reach healthy cells and cancer cells alike, but they do most of their damage to the cancer cells. Unfortunately, some types of healthy cells divide as rapidly as cancer cells and are badly damaged as well. Such cells are found in bone marrow, the lining of the digestive tract, and hair follicles, so chemotherapy patients often lose their hair and are susceptible to infection....

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