Human papillomavirus infection (HPV) causes cervical cancer. Older studies said it caused more than 90% of cases. The World Health Organization's "World Cancer Report" now says, "Thirteen sexually transmitted mucosal human papillomavirus (HPV) subtypes are established human carcinogens. Together, they are responsible for all cervical cancer cases globally." Vaccination against HPV before a young person becomes sexually active can prevent cervical cancer. Both girls and boys should get the HPV vaccine. The vaccine usually requires two doses. Some parents who are social conservatives say it is a bad idea to give young people the HPV vaccine. They think it will encourage young, people to have sex before they are married. They say their daughters won't get cervical cancer because their daughters will not have sex until they are married. The parents are not worried that their daughters could die of cervical cancer. Other parents think vaccines are more dangerous than the diseases they prevent. Vaccination is uncommon in developing countries due to the high cost or because there are few doctors and nurses to give the vaccines. 