Those pesky viruses can make us all sick, but one day could be engineered to defeat cancer. Cancer cells have one trait that may leave them open to attack. They aren't good at killing off viral infections, hence, at least in theory, you could use a virus to kill cancer cells without affecting the patient.
Some of the efforts to use viruses to defeat cancer date back to 1951. A 4-year old with leukemia caught chicken pox. As a result the leukemia went into remission, but later returned once the chicken pox was gone, unfortunately the child later died. Continued efforts to use these techniques went on until the 1960s and after this point, other methods began to be investigated. Since the 1960s much has happened in science, with an understanding in the genetics and workings of viruses and cancer.