Hint: An oncogene is a gene with the ability to cause cancer. These genes are often mutated or overexpressed in tumour cells. When essential functions are altered or malfunctioning, most normal cells will undergo programmed rapid cell death.
Complete answer: Carcinogen, any of a variety of agents capable of causing cancer in humans. Chemical carcinogens (including those derived from biological sources), physical carcinogens, and oncogenic (cancer-causing) viruses are the three main types. An oncovirus, also known as an oncogenic virus, is a virus that can cause cancer. The word "oncornaviruses" was coined during research on acutely transforming retroviruses in the 1950s–60s, when the term "oncornaviruses" was used to describe their RNA virus origin. With the letters "RNA" deleted, it now refers to any virus that causes cancer and has a DNA or RNA genome, and is synonymous with "tumour virus" or "cancer virus". A variety of viruses, including oncogenic viruses, are suspected of causing cancer in animals, including humans. Human papillomaviruses, Epstein-Barr virus, and hepatitis B virus are all examples of viruses with DNA genomes. The retrovirus (a type of RNA virus) human T-cell leukaemia virus type I (HTLV-I) has been related to tumour formation in humans. These cancers could be easily avoided with vaccination (for example, papillomavirus vaccines), detected with easy blood tests, and treated with less-toxic antiviral compounds.
Note: According to the World Health Organization's International Agency for Research on Cancer, infection caused 17.8 percent of human cancers in 2002, with one of seven viruses accounting for 11.9 percent. A 2020 analysis of 2,658 samples from 38 different forms of cancer discovered that $16\%$ were linked to a virus.