Pap smears catch HPV early.
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Pap smears catch HPV early.(GETTY IMAGES)HPV, herpes, and chlamydia are the sexually transmitted diseases (STDs) for which sexually active people are most at risk.

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.

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Unfortunately, protecting yourself from these STDs isn't always as simple as reaching for a condom, because while they do significantly decrease the risk of transmitting these infections, they might not cover all the areas where they reside.

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.