LOVE WINS: CDC Official: Syphilis Rates Among Gay Men Highest Since Before Start Of HIV Epidemic. [archive] The article helpfully quotes from a CDC document on syphilis:

“While rates have increased among both men and women, men account for more than 90 percent of all primary and secondary syphilis cases,” the fact sheet says.

“Men who have sex with men (MSM) account for 83 percent of male cases where the sex of the sex partner is known,” the fact sheet states. “Primary and secondary syphilis are the most infectious stages of the disease, and if not adequately treated, can lead to long-term infection, which can cause visual impairment and stroke.”

“Syphilis infection can also place a person at increased risk for acquiring or transmitting HIV infection,” it says. “Available surveillance data indicate that an average of half of MSM who have syphilis are also infected with HIV.”