They are found in cervical and penile lesions. Infection precedes disease. Animal papillomaviruses have been shown to cause tumors in natural infections and experimentally. Not everyone infected with these types will develop premalignant disease and even fewer will go on to the malignant stage. Additional factors, apart from infection, involved in disease development and progression are unknown.
Sexual contact is the most common form of transmission.
Highest incidence is in sexually active people in the 18 to 30 age group. In premalignant cells the viral DNA is separate from the chromosomal DNA and is replicating and producing infectious virus, while in most malignant cells, the viral DNA is stably integrated into the chromosomal DNA, resulting in no viral particle production. It is not clear if integration is a prerequisite for conversion to a malignant phenotype. There is an association between the rate of the carrier state and hepatocellular carcinoma. An additional factor is that in these countries the virus is transmitted to the neonate, at birth, or early in life through close contact with the mother.