The abnormal cell growths penetrate tissue in the body, and they can spread through the blood and lymph nodes, notes the National Cancer Institute. Cancer forms when the genetic structure of a cell degrades and maintains old and defective cells. These inferior cells form into tumors.
Cancer is not contagious. Malignant tumors are cancerous, explains MedicineNet. Benign tumors are not considered cancer, and some cancers do not cause tumor growth. For example, leukemia is a cancer that forms in blood-forming tissues, notes the National Cancer Institute.Learn more about Cancer