the centers for disease control and prevention (cdc) did not recognize female-to-female transmission as a possible method of hiv infection until 1995. the cdc reported there is little data available regarding the risk of spreading sexually transmitted infections between women. however, the cdc states that pathogens such as metronidazole-resistant trichomoniasis, genotype-concordant hiv, human papillomavirus (hpv, which has been linked to nearly all cases of cervical cancer) and syphilis can be spread through sexual contact between women. while the rates of these pathologies are unknown, one study showed 30% of lesbians and bisexual women had medical histories with sexually transmitted infections. this does not mean that sexually active lesbians are exposed to higher health risks than the general population. health canada noted "the prevalence of all types of hpv (cancer and non-cancer-causing) in different groups of canadian women ranges from 20%–33%" and an american university study found 60% of sexually active females were infected with hpv at some time within a three-year period.