The Boston Public Health Commission released 1999 statistics showing 2 percent of the city’s 15- to 19-year-olds have chlamydia. Boston’s minority girls were reported to have infection rates of almost 6 percent, and a 1998 Journal of the American Medical Association study of sexually active teenage girls in Baltimore found infection rates approaching 30 percent. In this era of sex education and ample health information, teen-agers in the U.S. probably are more aware of the dangers of sexually transmitted diseases than any generation before them, but they know little about some STDs. Consider Chlamydia trachomatis, the most common bacterial STD in the developed world. Since infection with Chlamydia is often silent, it goes mostly undetected even though it is the leading cause of preventable infertility in this country. In the Jan. 30, 2001, Proceedings of the National Academy of Sciences, Michael Starnbach, Harvard Medical School assistant professor, and colleagues presented data that advance researchers’ understanding of this mysterious pathogen and, at the same time, move them closer to developing a vaccine.