What is cancer? Cancer is a general term for a group of diseases that occur when there is an uncontrolled growth of abnormal cells in one or more organs or tissues of the body. Cancer is a serious disease and is one of the leading causes of death globally, according to the World Health Organization (Source: WHO). Common types of cancer include: Bladder cancer Breast cancer Colorectal cancer Endometrial cancer Leukemia Liver cancer Lung cancer Melanoma Non-Hodgkin lymphoma Prostate cancer Renal cell cancer (kidney cancer) Thyroid cancer Normally, cells in the body that are old or damaged stop dividing and die. These cells are replaced by healthy young cells. Cancer occurs when old or damaged cells continue to divide and multiply uncontrollably. This results in the development of a malignant tumor or other abnormalities that interfere with the functioning of the affected organ or tissue. Cancer cells crowd out or destroy healthy cells and, if left untreated, can continue to multiply and spread to other tissues and organs of the body—a process called metastasis. As cancer progresses, it interferes with vital processes and functions of the organ where it began, and of the organs where it has spread, such as the brain, lungs, bones and liver. There are two main types of cancer: Primary cancer refers to a malignant tumor that originates within a specific body tissue, such as primary lung cancer or primary breast cancer. Secondary cancer (metastatic cancer) is caused by a primary cancer that has begun in one tissue or organ of the body and spread to another site. For example, primary lung cancer or breast cancer can spread to the brain, causing secondary brain cancer. Cancer can metastasize by growing directly into nearby tissues and organs or by spreading through the circulatory system or lymphatic system to other areas of the body. Cancer can lead to life-threatening complications and be fatal, especially if it goes undetected and untreated. Prognosis of cancer varies depending on the type of cancer and the stage of advancement; your age, medical history, and coexisting conditions or diseases; and other factors. Seeking regular medical care offers the best chances of discovering cancer in its earliest, most curable stage. If you have cancer, following your treatment plan may help reduce your risk of serious complications.