Of more than academic interest is the fact that HIV and syphilis co-facilitate each other’s transmission. The reasons are listed on this slide. Estimates from a variety of studies suggest a 2-5 X higher rate of HIV transmission in the presence of ulcerative STDs, such as syphilis. The overall population attributable rate (PAR) of GUD for HIV is estimated at between 7 and 18% in the U.S., and 44 – 69% in the developing world.