Cancer Cancer is a group of diseases characterized by an uncontrolled growth of abnormal cells. If the spread of these abnormal cells is not controlled, cancer can cause death. Most cancers take the form of tumors, although not all tumors are cancers. A tumor is simply a mass of new tissue that serves no physiological purpose. It can be benign, like a wart, or malignant, like cancer. Benign tumors are made up of cells similar to the surrounding normal cells and are enclosed in a membrane that prevents them from penetrating neighboring tissues. They are dangerous only if their physical presence interferes with bodily functions. A malignant tumor, or cancer, is cap