SAN DIEGO -- Five state health investigators are joining in to help stem the spread of syphilis is San Diego County, where there's been a 1,100 percent rise in syphilis cases between 2000 and 2007.

California communicable disease control chief Douglas Hatch said, "San Diego's increase is a cause for concern because we're just not understanding why it's being transmitted in the frequency that we're seeing."

Most of the syphilis infections in San Diego, and nationwide, involve male homosexual encounters.

San Diego County syphilis cases went from 28 in 2000 to 340 last year, much higher that the other large urban counties of San Francisco, Los Angeles, Orange and Alameda.

The state team dispatched to San Diego County will contact people diagnosed with syphilis to locate their sexual partners so they can be tested.

Last month, the county health agency launched a radio and television ad campaign in an effort to reverse a spike in syphilis infections and other sexually transmitted diseases.