Population-based study on the prevalence of and risk factors for human papillomavirus infection in Qujing of Yunnan province, Southwest China oleh Lu-lu Sun, Qiong Jin, Hui Li, Xian-rong Zhou, Zhi-qin Song, Xue-mei Cheng, Tao Tao, Bing Liang, Lin Xu, Yu-rong Wang, Yan Zhen, Ji-wen He, Keng Shen