There are more than one hundred strains of HPV in existence, thirteen of which cause cervical, anal, oral, and penile cancer—today, around 90 percent of all sexually active adults become infected with at least one strain during their lifetimes. Throughout the eighties, using HeLa and other cells, scientists studied HPV infection and how it causes cancer. They learned that HPV inserts its DNA into the DNA of the host cell, where it produces proteins that lead to cancer. They also found that when they blocked the HPV DNA, cervical cancer
