Preliminary data from the Centers for Disease Control and Prevention, presented at the National STD Prevention Conference in Chicago Wednesday, show the national rate of primary and secondary syphilis -- the most infectious stages of the disease -- increased 16 percent between 2006 and 2007.

This overall increase was driven by continued increases among males -- from 5.7 per 100,000 in 2006 to 6.4 per 100,000 in 2007, the report said.

The rate among females also increased between 2006 and 2007, from 1.0 to 1.1 cases per 100,000 population. The reasons for the third consecutive annual increase among females are still being examined, but this emerging trend deepens concerns about a potential resurgence of syphilis among women, after more than a decade of declining rates.

Reported syphilis rates among African-American men increased 25 percent from 2006 and 2007 -- 17.1 to 21.5 cases per 100,000 population. The rate among African-American women rose 12 percent from 2006 and 2007 -- 4.8 to 5.4 cases per 100,000 population, the report said.