Virus does not cause, directly, cancer. Only virus eradication, however, can cure it. Virus kills cells. When stem cells are hit, tumor is formed as a protection (against Cachexia) mechanism. For more details see presentations from professor Zajicek on you tube. It assumed that stem cell secret a substance that sustain tissue. Tumor partially / inefficiently compensates for the created deficiency. If virus load increases, deficiency grows and so does the tumor.