Viruses are widely present in nature and can infect eukaryotic cells, producing a variety of biological effects. Several viruses are capable of coexisting with human cells over the long term, and it has been proven that these viruses possess carcinogenic properties. Radiation therapy is a common method used for treating tumors. Under the influence of radiation, infected tumor cells and uninfected tumor cells exhibit different pathological characteristics and treatment outcomes. Herein, in this chapter, we discuss the impact of five viruses on tumor radiation therapy, including coronavirus disease-2019 (COVID-19), hepatitis B virus (HBV), human papillomavirus (HPV), monkeypox virus, and avian influenza virus. We hope that by summarizing the relationship between viral replication and tumor radiation therapy, we can provide novel insight for future treatments.