HPV is the most common sexually transmitted infection. Those who are infected often have no symptoms and pass it on to their partners through genital contact during vaginal and anal sex. It can also be transmitted during oral sex and, more rarely, during deep kissing through saliva.
There are more than 100 strains of the virus. Some cause genital warts, but others can result in cell changes that decades later can become cancerous. Each strain is identified by a number; oral and cervical cancers are caused by HPV sub-types 16 and 18.
HPV can also cause cancers of the vulva, vagina and penis, and there is some evidence it is associated with lung cancers.