SAN FRANCISCO (AP) — The syphilis epidemic in San Francisco appears to be growing unabated, with the city on track to report 750 new cases by year’s end, up 50 percent from 2002, the city’s public health department has announced.

The vast majority of cases have been reported among gay and bisexual men who have had unprotected sex with multiple partners, male and female, health officials reported.

About two-thirds of the new cases are in people already infected with HIV, which is particularly alarming since it suggests many people are having unprotected sex despite knowing that they carry AIDS. Both diseases are transmitted sexually, but only syphilis is curable.

“Many people have lost their awareness or appreciation of syphilis as a health problem,” said Dr. Jeffrey Klausner, who directs the department’s sexually transmitted disease unit, which recorded 494 new syphilis cases last year.

Too many infected people fail to tell even their own doctors about their sexual practices — and therefore don’t get tested for syphilis or HIV — allowing their own health to suffer and the diseases to spread, Klausner said.

In response, the department and AIDS prevention groups said they are renewing efforts to build awareness about syphilis. The plan includes making syphilis testing easier and more accessible, and educating both gays and health providers about the need to perform the tests.

Syphilis begins as a sore and develops into a rash. It can be treated with penicillin, but if left untreated can cause permanent damage to vital organs with resultant blindness, deafness, dementia and ruptured blood vessels. People with HIV are even more susceptible.

Part of the problem is social stigma, which discourages some people from even asking their doctors for such tests. San Francisco has sought to overcome that with an online syphilis testing program that ensures anonymity. City residents can sign up on the Web, then get their blood drawn anonymously and for free at a number of clinics in the city, then see their results on the site, identified only by a confidential number.

The idea is to mobilize gay and bisexual men, encouraging them not to be ashamed, to get themselves tested and to stop spreading these diseases, said Les Pappas, a social marketing executive.

Pappas was part of the team that created the Healthy Penis 2003 campaign, which includes humorous cartoon advertisements appearing on city bus shelters, train stations and billboards.

The ads were barred from such visible locations until the campaign agreed to clothe one of the penis mascots in a trench coat and partially hide another behind a cartoon newspaper.