This question is often raised by cancer patients or their families, and for many years, medical doctors believed that human cancer is not contagious. Some viral infections associated with cancer development (e.g., human papillomavirus, hepatitis B virus, and EBV), although associated with the development of cervical cancer, liver cancer, nasopharyngeal cancer, and lymphoma, no evidence has been seen to date that these cancers can be directly transmitted to others. The incidence of cancer has also not been seen to be higher among medical personnel working in oncology consultation and treatment than among those working in other jobs. However, in 1980, it was discovered that human T-cell leukemia virus type I (HTLV-I) could be transmitted from one person to another, causing adult T-cell leukemia in the latter. Experimental animal studies have also found that nursing the offspring of mothers with a high incidence of breast cancer and raising the offspring of mothers with low cancer resulted in a significantly higher incidence of breast cancer in these offspring as they grew up, so that breast cancer was transmitted to the offspring from the mother’s milk as an infectious disease. Epidemiological investigations have found that some cancers are associated with insect transmission. For example, the high prevalence of EBV infection in African children with Burkitt’s lymphoma and the distribution of tumors consistent with malaria endemic areas, presumably both diseases are transmitted by mosquitoes as vectors. Viral antigens or antibodies can be found in some cancer patients. For example, in 80% of patients with cervical cancer, type B simple scar virus can be detected, and in 70% to 90% of patients with nasopharyngeal cancer, antibodies to EBV can be detected. All of these examples show that the occurrence of cancer is very closely related to viral infections, but there is insufficient evidence that these cancers are transmitted directly by viral infections. Recent studies have shown that there are not many different types of oncogenic viruses associated with human tumors. Due to the different types of viruses, they integrate into the human cell genome (DNA sequence) through viral RNA transduction or insertion mechanisms, or through viral DNA integration into the human cell genome, leading to activation of proto-oncogenes and/or inactivation of oncogenes, causing cells to transform and persistently proliferate, thus forming tumors.