A severe form of syphilis known as neurosyphilis has been detected among HIV-positive gay and bisexual men, according to the U.S. Centers for Disease Control. The percentage of syphilis cases diagnosed in gay and bisexual man has risen from 7% of all U.S. cases in 2000 to 60% in 2005. Steep syphilis increases cause alarm about HIV as well because they are a sure sign of unprotected sex.