Human tumor viruses account for an estimated 12% to 20% of cancers worldwide. Viruses can lead to cancer by associating with host proteins, proliferating when the human immune system is weakened, and hijacking proliferating human cells. Compared to other viruses, human tumor viruses are unusual because they infect, but do not kill, their host cells. This allows human tumor viruses to establish persistent infections. https://asm.org/articles/2019/january/the-seven-viruses-that-cause-human-cancers
