Cancer is a group of diseases characterized by the uncontrolled growth and spread of abnormal cells in the body. It is a leading cause of death worldwide and can affect any part of the body. There are over 100 types of cancer, including breast cancer, lung cancer, prostate cancer, and colon cancer. Cancer occurs when the body's normal control mechanism stops working. Old cells do not die and instead form a mass of tissue called a tumor. Tumors can be benign (not cancerous) or malignant (cancerous). Malignant tumors can invade nearby tissues and spread to other parts of the body through the bloodstream or lymphatic system. Cancer is caused by a combination of genetic and environmental factors. Some factors that increase the risk of developing cancer include smoking, exposure to radiation and chemicals, obesity, and a family history of cancer. Symptoms of cancer vary depending on the type and stage of cancer. Common symptoms include fatigue, unexplained weight loss, pain, and changes in the skin or bowel habits. However, some cancers do not cause any symptoms in the early stages.