Cancer is a group of diseases where abnormal cells multiply at an uncontrollable rate, often destroying normal body tissue. Cancer can start in one part of the body, such as the lungs, brain, breasts, or colon, and then spread to other parts. It can form masses of tissue known as tumors, which when malignant can attack healthy tissue and form new tumors. This is known as metastasis. According to the World Health Organization, cancer is the second leading cause of death worldwide.