(Reuters) - Cancer of the back of the mouth and throat is on the rise, primarily because of more cases stemming from a viral infection called human papillomavirus (HPV), according to a U.S. study.The number of people who were diagnosed with HPV-related oral cancer in 2004 was triple the number diagnosed in 1988 -- due largely, researchers suspect, to changes in sexual behavior that have helped spread the virus.HPV is a very common sexually transmitted infection that can cause genital warts and certain cancers, including of the cervix, anus and penis."The whole relationship between HPV-related head and neck cancer completely changes our ideas of who is at risk, how to treat the cancer, the prognostics of the cancer, and prevention," said Maura Gillison, at the Ohio State University, who led the study, published in the Journal of Clinical Oncology.Gillison and her colleagues examined oral cancer tissue collected from 271 patients over a 20-year period.