Human papillomavirus (HPV)
The good news about HPV is that most of the roughly 100 strains are pretty harmless; only about 30 are sexually transmitted and just a few cause cervical cancer. Women who get regular Pap smears detect precancerous cells early while it's still easy to treat.
Herpes is a painful, recurrent infection for only a small minority of those infected. For others, herpes is an occasional skin disease, and many don't have outbreaks at all.
Chlamydia doesn't tend to have any symptoms and can be very destructive to reproductive and general health if not diagnosed and treated in time. Chlamydia is one of the most common causes of infertility, according to H. Hunter Handsfield, MD, an AIDS and STD expert at the University of Washington, in Seattle.
And you can't always tell from looking whether your partner has an STD, because there may be no visible symptoms. So in addition to using condoms, it's important to talk to your partner about his or her relationship history and sexual practices and to consider getting tested for STDs before starting a sexual relationship.