Cancer is a disease that uncontrollably spreads from some cells of the body to other regions of the body. In the human body, which consists of billions of cells, cancer may begin nearly everywhere. Normally, human body cells develop and multiply to produce new cells when the body requires them, employing a process called cell division. They die and fresh cells replace when cells get old or are damaged. It breaks down sometimes and aberrant or damaged cells develop and reproduce if they do not. These cells can become tumors that are tissue masses. Tumors may or may not be carcinogenic (benign). Cancer tumors expand to surrounding tissues or intrude into them and can go to remote areas of the body to develop new tumors (a process called metastasis). Malignant tumors also can be known as cancerous tumors. Many cancers cause strong tumors, but typically do not cause blood cancers, such as leukemia. Benign tumors do not spread or infect surrounding tissues. Usually, benign tumors do not regrow when they are removed, although sometimes malignant tumors do. However, occasionally benign tumors can be extremely big. Some can lead to severe symptoms or life-threatening conditions such as benign brain tumors.