Genital warts are caused by infection with certain types of HPV, a virus that infects the top layers of skin. There are more than 100 different types of HPV, but only a few of them cause genital warts. Two types of HPV are responsible for most cases of genital warts: types 6 and 11. Other types of HPV cause warts on other parts of the body such as the common warts that affect the hands, while still other HPV types can lead to cancers of the cervix, or less commonly, cancers of the external genitalia and anus.

HPV infection is spread through skin-to-skin contact. The types of the virus that cause genital warts are highly contagious and spread easily through sexual contact (anal, oral, or vaginal) with an infected person. It is estimated that about two-thirds of people who have sexual contact with someone who has genital warts develop the condition; warts can appear weeks or months afterwards but usually show up within three months of contact.