The tiny "bricks" our bodies are built of are called cells. A single one of these cells is too small to be seen without a microscope. Once in a while some cells somewhere in a person's body go on a rampage. They begin to grow and multiply very fast. In time they begin crowding other cells and stealing food the other cells need. They cause the disease which is called cancer.
Cancer used to be incurable. But now many people who have cancer get well. If it is discovered in time, the chances of curing it are very good. One reason why everyone should go to a doctor for a yearly checkup is to make sure no cancer cells are starting to grow.
Many doctors are at work in hospitals, universities, and laboratories trying to learn what causes cancer. When the cause is known, doctors will be better able to prevent and cure cancer.