One in 5 cancers worldwide are caused by infection, most of which are caused by viruses.  By understanding how a virus can turn a healthy cell into a cancerous cell, we gain critical insights into how the growth of any cancer cell is controlled.  Our laboratory discovered two of the seven known human cancer viruses.  In 1994, we isolated Kaposi sarcoma herpesvirus (KSHV or HHV8) that causes cancers among immune suppressed patients, especially persons with AIDS.  KSHV is a scourge in Africa and the Middle East, and it became established in Xinjiang and Western China in historical times probably by spread through trade routes.   In 2008, we discovered Merkel cell polyomavirus, which causes Merkel cell carcinoma, one the most aggressive forms of skin cancer.  This previously untreatable cancer is now often controlled by immune therapies that activate immune responses against this virus.   New cancer viruses can be discovered by screening for viral DNA, RNA and proteins in cancers, which opens new vistas in the diagnosis, treatment and prevention of viral cancers.