Genital HPV is spread through contact with (touching) the skin of someone who has an HPV infection. Contact includes vaginal, anal, and oral sex.
High-risk HPVs cause several types of cancer.
Cervical cancer: Virtually all cases of cervical cancer are caused by HPV, and just two HPV types, 16 and 18, are responsible for about 70% of all cases.
Anal cancer: About 95% of anal cancers are caused by HPV. Most of these are caused by HPV type 16.
Oropharyngeal cancers (cancers of the middle part of the throat, including the soft palate, the base of the tongue, and the tonsils): About 70% of oropharyngeal cancers are caused by HPV.
In the United States, more than half of cancers diagnosed in the oropharynx are linked to HPV type 16.
Rarer cancers: HPV causes about 65% of vaginal cancers, 50% of vulvar cancers, and 35% of penile cancers. Most of these are caused by HPV type 16.
High-risk HPV types cause approximately 5% of all cancers worldwide. In the United States, high-risk HPV types cause approximately 3% of all cancer cases among women and 2% of all cancer cases among men.
HPV infects epithelial cells. These cells, which are organized in layers, cover the inside and outside surfaces of the body, including the skin, the throat, the genital tract, and the anus.
Once HPV enters an epithelial cell, the virus begins to make the proteins it encodes.
Two of the proteins made by high-risk HPVs (E6 and E7) interfere with cell functions that normally prevent excessive growth, helping the cell to grow in an uncontrolled manner and to avoid cell death.
Many times, these infected cells are recognized by the immune system and eliminated. Sometimes, however, these infected cells are not destroyed, and a persistent infection results.
As the persistently infected cells continue to grow, they may develop mutations in cellular genes that promote even more abnormal cell growth.
This will lead to the formation of an area of precancerous cells and, ultimately, a cancerous tumor.
Some factors may increase the risk that an infection with a high-risk HPV type will persist and possibly develop into cancer. These include:
- Smoking or chewing tobacco (for increased risk of oropharyngeal cancer)
- Having a weakened immune system
- Having many children (for increased risk of cervical cancer)
- Long-term oral contraceptive use (for increased risk of cervical cancer)
- Poor oral hygiene (for increased risk of oropharyngeal cancer)
- Chronic inflammation
Researchers believe that it can take between 10 and 30 years from the time of an initial HPV infection until a tumor forms.
However, even when severely abnormal cells are seen on the cervix (a condition called cervical intraepithelial neoplasia 3, or CIN3), these do not always lead to cancer.
The percentage of CIN3 lesions that progress to invasive cervical cancer has been estimated to be 50% or less.
News source: National Cancer Institute. The content is edited for length and style purposes.
Figure legend: This Knowridge.com image is credited to Pan American Health Organization / Flickr.