Syphilis is believed to have infected 12 million people in 1999 with greater than 90% of cases in the developing world. It affects between 700,000 and 1.6 million pregnancies a year resulting in spontaneous abortions, stillbirths, and congenital syphilis. In Sub-Saharan Africa syphilis contributes to approximately 20% of perinatal deaths. Increased rates among heterosexuals have occurred in China and Russia since the 1990s. Syphilis increases the risk of HIV transmission by two to five times and co-infection is common (30–60% in a number of urban centers).