In early adolescence and early reproductive age, when squamous epithelial metaplasia occurs, infection with HPV virus can induce changes in newly transformed cells, and viral particles can integrate into human cell DNA. If the virus persists, it may lead to precancerous lesions, and later the cells lose their normal regulation and become cancerous. The time from HPV infection to cancer progression varies. Sixty percent or more of mild atypical hyperplasia will resolve spontaneously, and only about 10% develop into moderate or severe atypical hyperplasia in 2 to 4 years. In some cases, moderate or severe atypical hyperplasia may not need to go through mild atypical hyperplasia. Less than 50% of severe atypical hyperplasia may progress to invasive carcinoma, and younger women have an even lower chance of developing invasive carcinoma. Usually mild atypical hyperplasia develops into cancer only after a natural progression of 10-20 years. Therefore, cervical cancer is a preventable cancer at a relatively early stage, providing the conditions for screening.