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Human papillomavirus, or HPV, is the most common sexually transmitted infection. Nearly 80 million Americans have the infection, and about 14 million become newly infected every year. In fact, most sexually active people contract HPV at some point in their lives. The virus spreads easily through skin-to-skin sexual contact.

HPV doesn’t always cause symptoms, and many people with the virus don’t know they have it. More than 90 percent of all new HPV infections become undetectable within two years, even without treatment. Yet some HPV infections can stay in the body and lead to complications, including genital and anal warts. These warts may be small or large, flat or raised; they may emerge singly or in a cluster and be cauliflower shaped. They appear most often on the genitals and anus. HPV infection may also cause more serious conditions, such as certain types of cancer.

Of the more than 150 strains of HPV, 40 affect the genital area, but most don’t pose a serious risk to health. A person can have more than one HPV strain at a time. Strains are identified by number and fall into either of the following two categories.

Low-Risk Human Papillomavirus

Most low-risk genital HPV strains doesn’t cause symptoms and disappears when the body builds immunity to the virus. These strains have no association with cancer but can lead to genital warts.

In addition to the vulva and the penis, warts may appear on the cervix or vagina, the scrotum, in and around the anus, or in the mouth and throat. Two strains of HPV, types 6 and 11, cause 90 percent of these warts. Only about 1 percent of sexually active Americans have noticeable genital warts, which require treatment to reduce the spread to other genital areas and to sexual partners.

Some low-risk HPV strains can cause mild cervical and anal dysplasia, abnormal changes in the cells on the surface of these areas. These changes are not precancerous.

High-Risk Human Papillomavirus

Infection with high-risk HPV can lead to more extensive cervical and anal dysplasia and eventually certain types of cancer. There are at least 14 high-risk strains of HPV, but two—types 16 and 18—cause the majority of HPV-related cancers, including those involving the cervix, vagina, vulva, penis, and anus. High-risk HPV strains can also lead to cancers of the throat, tongue, and tonsils, known as oropharyngeal cancer.

Researchers believe that HPV infections of the mouth and throat may be caused by oral sexual contact with someone who has an active high-risk infection. It’s important to remember, however, that for most people, the immune system suppresses HPV within two years. Although infection with high-risk HPV strains can cause cancer, most people with these strains don’t develop cancer.