There are several clinical trials underway to test this therapy (e.g. NCT03866525, NCT01503177, NCT04695327 ). The term is oncolytic virotherapy. Its not the virus that kills the cancer cells, it is the immune system that kills the virus infected cells. There are plenty of viruses that are more or less harmless for humans. Dr. Halassy for example used a measles virus that is also being used in vaccines.