I'm an oncologist. I specialize in cancer. It's a group of diseases in which abnormal cells in your body replicate and destroy healthy tissue. If it's localized, it may manifest in a tumor -- a mass of abnormal cells. Depending on the location, it may be possible to remove the tumor manually. Cancer usually becomes fatal when tumors can't be removed or go undetected, allowing the abnormal cells to break off and spread to other places in your body. It's a very difficult and unfortunately not uncommon group of diseases.