STDs are sexually transmitted diseases. This means they are most often -- but not exclusively -- spread by sexual intercourse. HIV, chlamydia, genital herpes, genital warts, gonorrhea, some forms of hepatitis, syphilis, and trichomoniasis are STDs.

STDs used to be called venereal diseases or VD. They are among the most common contagious diseases. It's estimated that 25% of all Americans have an incurable STD. Each year, 20 million new cases are reported; half of these infections are among people ages 15 to 24 and they can have long-term consequences.

STDs are serious illnesses that require treatment. Some STDs, such as HIV, cannot be cured and can be deadly. By learning more about STDs, you can learn ways to protect yourself.

You can get an STD from vaginal, anal, or oral sex. You can also be infected with trichomoniasis through contact with damp or moist objects such as towels, wet clothing, or toilet seats, although it is more commonly spread by sexual contact. You are at high risk if:

• You have more than one sex partner
• You have sex with someone who has had many partners
• You don't use a condom when having sex
• You share needles when injecting intravenous drugs
• You trade sex for money or drugs

HIV and herpes are chronic conditions that can be managed but not cured. Hepatitis B also may become chronic but can be managed. You may not realize you have certain STDs until you have damage to your reproductive organs (rendering you infertile), your vision, your heart, or other organs. Having an STD may weaken the immune system, leaving you more vulnerable to other infections. Pelvic inflammatory disease (PID) is a complication of gonorrhea and chlamydia that can leave women unable to have children. It can even kill you. If you pass an STD to your newborn child, the baby may suffer permanent harm or death.

What Causes STDs?

STDs include just about every kind of infection. Bacterial STDs include chlamydia, gonorrhea, and syphilis. Viral STDs include HIV, genital herpes, genital warts (HPV), and hepatitis B. Trichomoniasis is caused by a parasite.

The germs that cause STDs to hide in semen, blood, vaginal secretions, and sometimes saliva. Most of the organisms are spread by vaginal, anal, or oral sex, but some, such as those that cause genital herpes and genital warts, may be spread through skin contact. You can get hepatitis B by sharing personal items, such as toothbrushes or razors, with someone who has it.