A risk-factor analysis of reported cases showed that the proportion of intravenous drug users infected with HIV-1 increased from 1.7% (13/772) in 2002, to 8.1% (70/862) in 2003, to 41.3% (628/1520) in 2004, to 72.4% (2461/3399) in 2005, and dropped to 68.6% (2017/2974) in 2006. A molecular epidemiological study showed that more than 95% of intravenous drug users with newly diagnosed HIV-1 in 2004 and 2005 were infected with CRF07_BC, a circulating recombinant form of subtypes B' and C. Previously, several studies suggested that CRF07_BC originated in China's Yunnan province as a mix of subtype B' from Thailand and subtype C from India. The subtype is believed to have moved to Xinjiang in China's northwest along a major heroin-trafficking route.