A considerable amount of research is presently under way around the world to perfect effective cancer vaccines. At first glance, this may sound surprising, since cancer is not an infectious disease caused by invading germs, as are malaria, tuberculosis, influenza, and AIDS. No one "catches" cancer from inhaling germ-infested air or being bitten by a mosquito carrying a deadly microbe.

Instead, cancer consists of clusters of abnormal cells that appear and multiply within a person's body. With some exceptions (such as brain cells), most of the many different kinds of cells in the body divide now and then to produce fresh tissue. The cells that make up the skin are a clear example. About once a month, skin cells divide and the old cells fall away to make way for the new cells. Somehow, in ways scientists do not yet fully understand...