Given that 85% of cancers are caused by viruses, is this the reason chemotherapy often fails to cure cancer, because the usual cytotoxic drugs and radiation treatment fail to eliminate chronic viruses that cause a return and spread of malignant cells?

This is indeed the case. Not being aware that viruses are a likely culprit in causing cancer to begin with, the medical mainstream does not think at all about antiviral therapy as an adjunct or main focus as it should be. So this puts the patient on a potential treadmill where chemotherapy might successfully reduce and then eliminate malignant transformation of cells in the body to produce a tumor, but the cancer will reoccur as the virus bounces back and then produces new transformed cells once the chemotherapy treatments are over with. This also accounts for subsequent spread and emergence of cancer in new locations, sometimes misconstrued as an unrelated cancer event when it is really the original virus showing its continued presence with an impact on the body that becomes visible. The solution is at hand in using divine healing requests with sufficient specificity to fully empower the divine realm to eradicate harmful chronic viruses from the body. That will both prevent and cure ongoing malignancy if the full protocols are implemented, with the current requests for dealing with viruses as well as abnormal cells of the body, to revert them back to a normal configuration.