The number of Americans with three common sexually transmitted diseases has gone up for the second year in a row.

Syphilis cases rose 14% from 2005 to 2006, with a majority of cases among homosexual men. Syphilis can be treated in its early stages, but can lead to serious complications -- even death -- if not treated.

The rates of Chlamydia, a disease that can lead to infertility in women, went up over 5%.

Gonorrhea cases were up 5 1/2%, with African Americans reporting the disease 18 times more than white people.

CDC officials say many STD cases remain undiagnosed, with as many as 19 million new infections each year.

Researchers are concerned because STD's not only come with their own complications, they also make people with HIV more infectious.

Experts say the increase may be due to better detection methods, but may also stem from riskier sexual behavior among gay men.