There are over 200 types of cancer. They are usually diagnosed when a person has symptoms and see a doctor. Tests are taken and the doctor can usually tell where in the body the cancer is. Very occasionally, however, a person can be diagnosed with cancer, but the doctor does not know where in the body the cancer is. This happens when secondary cancers (cancers that have spread from the original tumour) have been found, but that the doctor cannot find the original tumour itself.