Human papillomavirus (HPV) is a DNA virus that infects epithelial cells and can cause cervical cancer. There are over 100 strains of HPV, with about 30 having the potential to transform cells and cause cancer. HPV is the most common sexually transmitted infection in the US, transmitted through unprotected sex. While most HPV infections clear on their own, persistent infections can lead to precancerous lesions and cervical cancer over time if left untreated. Diagnosis is usually based on abnormal Pap test results, and vaccination prior to sexual activity provides 90-95% protection against the HPV strains most likely to cause cancer.